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.
`!strings.Contains`?

So you don't want this pkg to be in the filepath, which means you're getting the first package that called this function. OK.