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.
Yes, "should", not "won't". If it blows up, something's wrong and should be tracked down.