cmd/go: include cfg.BuildModReason in 'import lookup disabled' errors

This location was missed in CL 204521.

Updates #33326
Updates #33848

Change-Id: I0ece6d9b37548d8abb54f79c69be5548a0428c76
Reviewed-on: https://go-review.googlesource.com/c/go/+/210341
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/modload/import.go

@@ -183,13 +183,14 @@ func Import(path string) (m module.Version, dir string, err error) {
 	// Look up module containing the package, for addition to the build list.
 	// Goal is to determine the module, download it to dir, and return m, dir, ErrMissing.
 	if cfg.BuildMod == "readonly" {
-		if pathIsStd {
-			// 'import lookup disabled' would be confusing for standard-library paths,
-			// since the user probably isn't expecting us to look up a module for
-			// those anyway.
-			return module.Version{}, "", &ImportMissingError{Path: path}
+		var queryErr error
+		if !pathIsStd {
+			if cfg.BuildModReason == "" {
+				queryErr = fmt.Errorf("import lookup disabled by -mod=%s", cfg.BuildMod)
+			}
+			queryErr = fmt.Errorf("import lookup disabled by -mod=%s\n\t(%s)", cfg.BuildMod, cfg.BuildModReason)
 		}
-		return module.Version{}, "", fmt.Errorf("import lookup disabled by -mod=%s", cfg.BuildMod)
+		return module.Version{}, "", &ImportMissingError{Path: path, QueryErr: queryErr}
 	}
 	if modRoot == "" && !allowMissingModuleImports {
 		return module.Version{}, "", &ImportMissingError{

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

@@ -2,7 +2,7 @@
 # to individual missing packages.
 # Verifies golang.org/issue/34829.
 go list -mod=readonly -e -deps -f '{{if .Error}}{{.ImportPath}}: {{.Error}}{{end}}' .
-stdout 'example.com/missing: use.go:3:8: import lookup disabled by -mod=readonly'
+stdout 'example.com/missing: use.go:3:8: cannot find module providing package example.com/missing: import lookup disabled by -mod=readonly'
 
 -- go.mod --
 module example.com/m

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

@@ -6,13 +6,14 @@ env GOFLAGS=-mod=readonly
 go mod edit -fmt
 cp go.mod go.mod.empty
 ! go list all
-stderr 'import lookup disabled by -mod=readonly'
+stderr '^can''t load package: x.go:2:8: cannot find module providing package rsc\.io/quote: import lookup disabled by -mod=readonly'
 cmp go.mod go.mod.empty
 
 # -mod=readonly should be set implicitly if the go.mod file is read-only
 chmod 0400 go.mod
 env GOFLAGS=
 ! go list all
+stderr '^can''t load package: x.go:2:8: cannot find module providing package rsc\.io/quote: import lookup disabled by -mod=readonly\n\t\(go.mod file is read-only\.\)$'
 
 chmod 0600 go.mod
 env GOFLAGS=-mod=readonly