URL parser silently ignores the "path" portion

The documented URL format is `codecommit://profile@repository`, where "profile" is effectively placed as the username and "repository" as the hostname when following typical URL rules.

The parsing code uses [`urlparse`](https://github.com/aws/git-remote-codecommit/blob/5ed175190c584bef20eceb84c1eb5aaafec22cf7/git_remote_codecommit/__init__.py#L95) and then inspects the various parts, but it totally ignores the path portion of the URL.

The tool will therefore accept a URL like `codecommit://profile@repository/any/other/nonsense`, but will silently discard the path and just treat it the same as `codecommit://profile@repository`, therefore still cloning the entire repository.

Since the path portion has no meaning, I'd suggest making it an error to specify a non-empty path, explicitly stating that a path is not allowed. Because Git repositories conceptually contain a filesystem subtree, it's tempting for someone to think that a path portion will clone a subdirectory of the repository, even though that's not technically possible due to how Git works. Returning an error would therefore give better feedback to someone who has made that mistake.