cmd/go/internal/modload: make AmbiguousImportError an ImportPathError

AmbiguousImportErrors will now be formatted like other ImportPathErrors:
this means that now the ambiguously imported package won't be printed
twice. Whereas the error message looked like the following:

	can't load package: package example.com/m/importy: ambiguous import: found package example.com/m/importy in multiple directories:
		$WORK/importy
		$WORK/vendor/example.com/m/importy

It now looks like this:

	can't load package: ambiguous import: found package example.com/m/importy in multiple directories:
		$WORK/importy
		$WORK/vendor/example.com/m/importy

Change-Id: I52a2074a6b3f5eb7d78d331d0852b7ea6b3735e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/221457
Run-TryBot: Michael Matloob <[email protected]>
Run-TryBot: Bryan C. Mills <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

src/cmd/go/internal/modload/import.go

@@ -62,19 +62,23 @@ func (e *ImportMissingError) ImportPath() string {
 // modules in the build list, or found in both the main module and its vendor
 // directory.
 type AmbiguousImportError struct {
-	ImportPath string
+	importPath string
 	Dirs       []string
 	Modules    []module.Version // Either empty or 1:1 with Dirs.
 }
 
+func (e *AmbiguousImportError) ImportPath() string {
+	return e.importPath
+}
+
 func (e *AmbiguousImportError) Error() string {
 	locType := "modules"
 	if len(e.Modules) == 0 {
 		locType = "directories"
 	}
 
 	var buf strings.Builder
-	fmt.Fprintf(&buf, "ambiguous import: found package %s in multiple %s:", e.ImportPath, locType)
+	fmt.Fprintf(&buf, "ambiguous import: found package %s in multiple %s:", e.importPath, locType)
 
 	for i, dir := range e.Dirs {
 		buf.WriteString("\n\t")
@@ -93,6 +97,8 @@ func (e *AmbiguousImportError) Error() string {
 	return buf.String()
 }
 
+var _ load.ImportPathError = &AmbiguousImportError{}
+
 // Import finds the module and directory in the build list
 // containing the package with the given import path.
 // The answer must be unique: Import returns an error
@@ -136,7 +142,7 @@ func Import(path string) (m module.Version, dir string, err error) {
 		mainDir, mainOK := dirInModule(path, targetPrefix, ModRoot(), true)
 		vendorDir, vendorOK := dirInModule(path, "", filepath.Join(ModRoot(), "vendor"), false)
 		if mainOK && vendorOK {
-			return module.Version{}, "", &AmbiguousImportError{ImportPath: path, Dirs: []string{mainDir, vendorDir}}
+			return module.Version{}, "", &AmbiguousImportError{importPath: path, Dirs: []string{mainDir, vendorDir}}
 		}
 		// Prefer to return main directory if there is one,
 		// Note that we're not checking that the package exists.
@@ -176,7 +182,7 @@ func Import(path string) (m module.Version, dir string, err error) {
 		return mods[0], dirs[0], nil
 	}
 	if len(mods) > 0 {
-		return module.Version{}, "", &AmbiguousImportError{ImportPath: path, Dirs: dirs, Modules: mods}
+		return module.Version{}, "", &AmbiguousImportError{importPath: path, Dirs: dirs, Modules: mods}
 	}
 
 	// Look up module containing the package, for addition to the build list.

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

@@ -11,12 +11,12 @@ go build ./importy
 
 # An import provided by both the main module and the vendor directory
 # should be flagged as an error only when -mod=vendor is set.
-# TODO: This error message is a bit redundant.
 mkdir vendor/example.com/m/importy
 cp $WORK/importy/importy.go vendor/example.com/m/importy/importy.go
 go build example.com/m/importy
 ! go build -mod=vendor example.com/m/importy
-stderr '^can.t load package: package example.com/m/importy: ambiguous import: found package example.com/m/importy in multiple directories:\n\t'$WORK'[/\\]importy\n\t'$WORK'[/\\]vendor[/\\]example.com[/\\]m[/\\]importy$'
+stderr '^can.t load package: ambiguous import: found package example.com/m/importy in multiple directories:\n\t'$WORK'[/\\]importy\n\t'$WORK'[/\\]vendor[/\\]example.com[/\\]m[/\\]importy$'
+
 
 -- $WORK/go.mod --
 module example.com/m