Fix incompatible map file system adapter.

This should make tests pass like before.
This essentially brings back the functionality of removed in
go-vcs/vcs/testing/mock_fs.go, except it's made private in this
package. That makes more sense since mapFS is only used for tests, and
GoDocVFS adapter is no longer necessary (besides, it was doing more
than its name suggested, which wasn't a good idea).