cipd: Implement local cache for ResolveVersion(...) call with tags.

go/src/infra/tools/cipd/local/fs.go

Index: go/src/infra/tools/cipd/local/fs.go
diff --git a/go/src/infra/tools/cipd/local/fs.go b/go/src/infra/tools/cipd/local/fs.go
index 266658ae3eafada75904d9d5e52ec3b16b65e3d7..5c7bfa3a79d897a7a8e03b474af98bf09f2c4bfb 100644
--- a/go/src/infra/tools/cipd/local/fs.go
+++ b/go/src/infra/tools/cipd/local/fs.go
@@ -81,8 +81,6 @@ type fsImplErr struct {
 	err error
 }
 
-// Root returns absolute path to a directory FileSystem operates in. All FS
-// actions are restricted to this directory.
 func (f *fsImplErr) Root() string                                                { return "" }
 func (f *fsImplErr) CwdRelToAbs(path string) (string, error)                     { return "", f.err }
 func (f *fsImplErr) RootRelToAbs(path string) (string, error)                    { return "", f.err }