cmd/go/internal/load: trim the " (test)" suffix from ImportStack paths

We were passing untrimmed paths to ModPackageModuleInfo, which was then failing
the build because it was asked to resolve an invalid path.

Fixes #26722

Change-Id: I043cc9c26f2188c5e005c0353620d9c55b339df9
Reviewed-on: https://go-review.googlesource.com/127795Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/internal/load/pkg.go

@@ -318,7 +318,8 @@ func (p *PackageError) Error() string {
 	return "package " + strings.Join(p.ImportStack, "\n\timports ") + ": " + p.Err
 }
 
-// An ImportStack is a stack of import paths.
+// An ImportStack is a stack of import paths, possibly with the suffix " (test)" appended.
+// TODO(bcmills): When the tree opens for 1.12, replace the suffixed string with a struct.
 type ImportStack []string
 
 func (s *ImportStack) Push(p string) {
@@ -986,7 +987,7 @@ func disallowInternal(srcDir string, p *Package, stk *ImportStack) *Package {
 		// p is in a module, so make it available based on the import path instead
 		// of the file path (https://golang.org/issue/23970).
 		parent := p.ImportPath[:i]
-		importer := (*stk)[len(*stk)-2]
+		importer := strings.TrimSuffix((*stk)[len(*stk)-2], " (test)")
 		if str.HasPathPrefix(importer, parent) {
 			return p
 		}
@@ -1039,7 +1040,7 @@ func disallowVendor(srcDir, path string, p *Package, stk *ImportStack) *Package
 		// but the usual vendor visibility check will not catch them
 		// because the module loader presents them with an ImportPath starting
 		// with "golang_org/" instead of "vendor/".
-		importer := (*stk)[len(*stk)-2]
+		importer := strings.TrimSuffix((*stk)[len(*stk)-2], " (test)")
 		if mod := ModPackageModuleInfo(importer); mod != nil {
 			dir := p.Dir
 			if relDir, err := filepath.Rel(p.Root, p.Dir); err == nil {

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

+env GO111MODULE=on
+
+cd a/
+go test
+stdout PASS
+
+-- a/go.mod --
+module github.com/user/a
+
+-- a/a.go --
+package a
+
+-- a/a_test.go --
+package a
+
+import "testing"
+
+func Test(t *testing.T) {}