doc: add glossary to module reference documentation

These terms will be defined throughout the document, and more terms
will be added. After drafting a few sections, it's clear that a
glossary will be useful. There are enough terms that it would be
overwhelming at the beginning.

Also, add anchors for each heading and add a couple more headings.

Updates #33637

Change-Id: I0017064f0b1e5e656dd280018ca0379484345df1
Reviewed-on: https://go-review.googlesource.com/c/go/+/206478
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Tyler Bui-Palsulich <tbp@google.com>

doc/modules.md

@@ -5,57 +5,209 @@
 }-->
 <!-- TODO(jayconrod): ensure golang.org/x/website can render Markdown or convert
 this document to HTML before Go 1.14. -->
-<!-- TODO(jayconrod): ensure Markdown renderer adds anchors or add them
-manually. -->
+<!-- TODO(jayconrod): ensure anchors work correctly after Markdown rendering -->
 
+<a id="introduction"></a>
 ## Introduction
 
-## Glossary
+<a id="modules-overview"></a>
+## Modules, packages, and versions
+
+<a id="versions"></a>
+### Versions
+
+<a id="major-version-suffixes"></a>
+### Major version suffixes
 
-## Packages, modules, and versions
+<a id="resolve-pkg-mod"></a>
+### Resolving a package to a module
 
-## go.mod files
+<a id="go.mod-files"></a>
+## `go.mod` files
 
-### go.mod file format
+<a id="go.mod-file-format"></a>
+### `go.mod` file format
 
+<a id="minimal-version-selection"></a>
 ### Minimal version selection (MVS)
 
+<a id="non-module-compat"></a>
 ### Compatibility with non-module repositories
 
+<a id="mod-commands"></a>
 ## Module-aware build commands
 
+<a id="enabling"></a>
 ### Enabling modules
 
+<a id="initializing"></a>
 ### Initializing modules
 
+<a id="build-commands"></a>
 ### Build commands
 
+<a id="vendoring"></a>
 ### Vendoring
 
+<a id="go-mod-download"></a>
 ### `go mod download`
 
+<a id="go-mod-verify"></a>
 ### `go mod verify`
 
+<a id="go-mod-edit"></a>
 ### `go mod edit`
 
+<a id="go-clean-modcache"></a>
 ### `go clean -modcache`
 
+<a id="commands-outside"></a>
 ### Module commands outside a module
 
+<a id="retrieving-modules"></a>
 ## Retrieving modules
 
+<a id="goproxy-protocol"></a>
 ### GOPROXY protocol
 
-### Module zip requirements
+<a id="communicating-with-proxies"></a>
+### Communicating with proxies
+
+<a id="communicating-with-vcs"></a>
+### Communicating with version control systems
+
+<a id="custom-import-paths"></a>
+### Custom import paths
+
+<!-- TODO(jayconrod): custom import paths, details of direct mode -->
 
-### Privacy
+<a id="path-constraints"></a>
+### File name and path constraints
 
+<a id="zip-format"></a>
+### Module zip format
+
+<a id="private-modules"></a>
 ### Private modules
 
+<a id="authenticating"></a>
 ## Authenticating modules
 
+<a id="go.sum-file-format"></a>
 ### go.sum file format
 
+<a id="checksum-database"></a>
 ### Checksum database
 
-### Privacy
+<a id="privacy"></a>
+## Privacy
+
+<a id="environment-variables"></a>
+## Environment variables
+
+<a id="glossary">
+## Glossary
+
+<a id="glos-build-list"></a>
+**build list:** The list of module versions that will be used for a build
+command such as `go build`, `go list`, or `go test`. The build list is
+determined from the [main module's](#glos-main-module) [`go.mod`
+file](#glos-go.mod-file) and `go.mod` files in transitively required modules
+using [minimal version selection](#glos-minimal-version-selection). The build
+list contains versions for all modules in the [module
+graph](#glos-module-graph), not just those relevant to a specific command.
+
+<a id="glos-go.mod-file"></a>
+**`go.mod` file:** The file that defines a module's path, requirements, and
+other metadata. Appears in the [module's root
+directory](#glos-module-root-directory). See the section on [`go.mod`
+files](#go.mod-files).
+
+<a id="glos-import-path"></a>
+**import path:** A string used to import a package in a Go source file.
+Synonymous with [package path](#glos-package-path).
+
+<a id="glos-main-module"></a>
+**main module:** The module in which the `go` command is invoked.
+
+<a id="glos-major-version"></a>
+**major version:** The first number in a semantic version (`1` in `v1.2.3`). In
+a release with incompatible changes, the major version must be incremented, and
+the minor and patch versions must be set to 0. Semantic versions with major
+version 0 are considered unstable.
+
+<a id="glos-major-version-suffix"></a>
+**major version suffix:** A module path suffix that matches the major version
+number. For example, `/v2` in `example.com/mod/v2`. Major version suffixes are
+required at `v2.0.0` and later and are not allowed at earlier versions. See
+the section on [Major version suffixes](#major-version-suffixes).
+
+<a id="glos-minimal-version-selection"></a>
+**minimal version selection (MVS):** The algorithm used to determine the
+versions of all modules that will be used in a build. See the section on
+[Minimal version selection](#minimal-version-selection) for details.
+
+<a id="glos-minor-version"></a>
+**minor version:** The second number in a semantic version (`2` in `v1.2.3`). In
+a release with new, backwards compatible functionality, the minor version must
+be incremented, and the patch version must be set to 0.
+
+<a id="glos-module"></a>
+**module:** A collection of packages that are released, versioned, and
+distributed together.
+
+<a id="glos-module-graph"></a>
+**module graph:** The directed graph of module requirements, rooted at the [main
+module](#glos-main-module). Each vertex in the graph is a module; each edge is a
+version from a `require` statement in a `go.mod` file (subject to `replace` and
+`exclude` statements in the main module's `go.mod` file.
+
+<a id="glos-module-path"></a>
+**module path:** A path that identifies a module and acts as a prefix for
+package import paths within the module. For example, `"golang.org/x/net"`.
+
+<a id="glos-module-root-directory"></a>
+**module root directory:** The directory that contains the `go.mod` file that
+defines a module.
+
+<a id="glos-package"></a>
+**package:** A collection of source files in the same directory that are
+compiled together. See the [Packages section](/ref/spec#Packages) in the Go
+Language Specification.
+
+<a id="glos-package-path"></a>
+**package path:** The path that uniquely identifies a package. A package path is
+a [module path](#glos-module-path) joined with a subdirectory within the module.
+For example `"golang.org/x/net/html"` is the package path for the package in the
+module `"golang.org/x/net"` in the `"html"` subdirectory. Synonym of
+[import path](#glos-import-path).
+
+<a id="glos-patch-version"></a>
+**patch version:** The third number in a semantic version (`3` in `v1.2.3`). In
+a release with no changes to the module's public interface, the patch version
+must be incremented.
+
+<a id="glos-pre-release-version"></a>
+**pre-release version:** A version with a dash followed by a series of
+dot-separated identifiers immediately following the patch version, for example,
+`v1.2.3-beta4`. Pre-release versions are considered unstable and are not
+assumed to be compatible with other versions. A pre-release version sorts before
+the corresponding release version: `v1.2.3-pre` comes before `v1.2.3`. See also
+[release version](#glos-release-version).
+
+<a id="glos-pseudo-version"></a>
+**pseudo-version:** A version that encodes a revision identifier (such as a Git
+commit hash) and a timestamp from a version control system. For example,
+`v0.0.0-20191109021931-daa7c04131f5`. Used for [compatibility with non-module
+repositories](#non-module-compat) and in other situations when a tagged
+version is not available.
+
+<a id="glos-release-version"></a>
+**release version:** A version without a pre-release suffix. For example,
+`v1.2.3`, not `v1.2.3-pre`. See also [pre-release
+version](#glos-pre-release-version).
+
+<a id="glos-version"></a>
+**version:** An identifier for an immutable snapshot of a module, written as the
+letter `v` followed by a semantic version. See the section on
+[Versions](#versions).