Add a fallback check -- if pkg/OS_ARCH for the lib-path variable works, ...

...then use that.

A relatively minor change, but one that kind of surprised me. If I've set lib-path with `gocode set`, it expects the exact library path, which is not usually what you want/provide. It's easier to give it the root and let it figure out pkg/OS_ARCH from that.

So, why not both? This patch shouldn't break anybody and should fix some people (like myself) who set GOPATH in a virtualenv-sort-of-way.