Add sub-package support for go get

Currently, it is not possible to go get a sub-package, i.e. go get <stash>/<project>/<repo>/<sub>/<directory>.

Changes required:

1. loosen the URL requirement for the import path (from <stash>/<project>/<repo> to <stash>/<project>/<repo>(/<directory>)*
2. assume everything after <repo> is a sub-directory path
3. return the same meta tag that would get returned for just go get <stash>/<project>/<repo>

What go get will do is hit the http://<stash>/<project>/<repo>/<sub>/<directory> page looking for the go-import meta-tag, find a tag with a import prefix for the whole repository, hit the prefixed page http://<stash>/<project>/<repo>, find the original meta-tag, check out the repository, and install the requested package.