Support certs/ that live within $GOPATH/src

This adds an additional search directory that supports calling
git repositories whose certs/ directory is not at $GOPATH/certs, but
rather at $GOPATH/pkg/path/certs. Subpackages such as pkg/path/server
work as well.
Would it make sense to stop this function recursing all the way to `/certs`? Unless that's the intended behavior.

Also, filepath.Clean will remove the end os.PathSeparator, but will also clean up the inside of the path if you want it to (I'm not sure that you do).