refactor(internal/librarian): do not derive API path for Go (#4406)

This change modifies Librarian to skip deriving the API path for Go
libraries when one is not explicitly provided.

Previously, if a library was not a veneer and had no APIs defined,
Librarian would append an empty API entry and attempt to derive its
path.

However, for Go, the library name does not contain the relevant
information needed to accurately derive the API path.

As a result, Go library with no API does not need `veneer: true`.

For #4276
Fixes #4279

Author: JoeWang1127

internal/librarian/golang/module_test.go

@@ -178,6 +178,16 @@ func TestFill(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "no API",
+			library: &config.Library{
+				Name: "auth",
+			},
+			want: &config.Library{
+				Name: "auth",
+				Go:   &config.GoModule{},
+			},
+		},
 	} {
 		t.Run(test.name, func(t *testing.T) {
 			got, err := Fill(test.library)

internal/librarian/library.go

@@ -191,7 +191,9 @@ func libraryOutput(language string, lib *config.Library, defaults *config.Defaul
 // applyDefaults applies language-specific derivations and fills defaults.
 func applyDefaults(language string, lib *config.Library, defaults *config.Default) (*config.Library, error) {
 	if !isVeneer(language, lib) {
-		if len(lib.APIs) == 0 {
+		if len(lib.APIs) == 0 && language != config.LanguageGo {
+			// Do not derive API path for Go because the library name
+			// doesn't contain relevant info.
 			lib.APIs = append(lib.APIs, &config.API{})
 		}
 		for _, api := range lib.APIs {
@@ -204,7 +206,11 @@ func applyDefaults(language string, lib *config.Library, defaults *config.Defaul
 		if isVeneer(language, lib) {
 			return nil, fmt.Errorf("veneer %q requires an explicit output path", lib.Name)
 		}
-		lib.Output = defaultOutput(language, lib.Name, lib.APIs[0].Path, defaults.Output)
+		var apiPath string
+		if len(lib.APIs) > 0 {
+			apiPath = lib.APIs[0].Path
+		}
+		lib.Output = defaultOutput(language, lib.Name, apiPath, defaults.Output)
 	}
 	return fillLibraryDefaults(language, fillDefaults(lib, defaults))
 }

internal/librarian/library_test.go

@@ -523,6 +523,11 @@ func TestPrepareLibrary(t *testing.T) {
 			wantOutput:  "src/generated/cloud/orgpolicy/v1",
 			wantAPIPath: "google/cloud/orgpolicy/v1",
 		},
+		{
+			name:       "Go lib without api path",
+			language:   config.LanguageGo,
+			wantOutput: "src/generated",
+		},
 	} {
 		t.Run(test.name, func(t *testing.T) {
 			lib := &config.Library{