cmd/go: make 'go mod why' require a main module

Updates #32027

Change-Id: Ifc9427f35188c3fd356917d8510f3e01866ebca8
Reviewed-on: https://go-review.googlesource.com/c/go/+/205065
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modget/get.go

@@ -373,6 +373,7 @@ func runGet(cmd *base.Command, args []string) {
 
 		default:
 			// The argument is a package or module path.
+			if modload.HasModRoot() {
 				if pkgs := modload.TargetPackages(path); len(pkgs) != 0 {
 					// The path is in the main module. Nothing to query.
 					if vers != "upgrade" && vers != "patch" {
@@ -380,6 +381,7 @@ func runGet(cmd *base.Command, args []string) {
 					}
 					continue
 				}
+			}
 
 			first := path
 			if i := strings.IndexByte(first, '/'); i >= 0 {

src/cmd/go/internal/modload/load.go

@@ -406,6 +406,10 @@ func loadAll(testAll bool) []string {
 // matching pattern, which may be relative to the working directory, under all
 // build tag settings.
 func TargetPackages(pattern string) []string {
+	// TargetPackages is relative to the main module, so ensure that the main
+	// module is a thing that can contain packages.
+	ModRoot()
+
 	return matchPackages(pattern, imports.AnyTags(), false, []module.Version{Target})
 }
 

src/cmd/go/testdata/script/mod_outside.txt

@@ -67,10 +67,9 @@ go mod graph
 ! stdout .
 ! stderr .
 
-# 'go mod why' should report that nothing is a dependency.
-go mod why -m example.com/version
-stdout 'does not need'
-
+# 'go mod why' should fail, since there is no main module to depend on anything.
+! go mod why -m example.com/version
+stderr 'cannot find main module'
 
 # 'go mod edit', 'go mod tidy', and 'go mod fmt' should fail:
 # there is no go.mod file to edit.