internal: add GetStdlibPaths to datasource

Updates b/149933479.

Change-Id: I8c34d62ce216aaa30c79b171981bd190de2eb936
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/747786
Reviewed-by: Julie Qiu <julieqiu@google.com>

internal/datasource.go

@@ -63,4 +63,6 @@ type DataSource interface {
 	Search(ctx context.Context, query string, limit, offset int) ([]*SearchResult, error)
 	// GetVersionMap returns the VersionMap corresponding to the provided modulePath and requestedVersion.
 	GetVersionMap(ctx context.Context, modulePath, requestedVersion string) (*VersionMap, error)
+	// GetStdlibPathsWithSuffix returns standard library paths with the given suffix.
+	GetStdlibPathsWithSuffix(ctx context.Context, suffix string) ([]string, error)
 }

internal/proxydatasource/datasource.go

@@ -444,3 +444,8 @@ func (ds *DataSource) GetPathInfo(ctx context.Context, path, inModulePath, inVer
 func (ds *DataSource) GetVersionMap(ctx context.Context, modulePath, version string) (*internal.VersionMap, error) {
 	return nil, nil
 }
+
+// GetStdlibPathsWithSuffix is unimplemented.
+func (ds *DataSource) GetStdlibPathsWithSuffix(ctx context.Context, suffix string) ([]string, error) {
+	return nil, nil
+}