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.
It's not exactly intended, but I think it's acceptable since this codepath is only meant for dev. Ideally it would stop at $GOPATH, but that would require figuring out which of the paths applies to this import.