Support deeper subdirectory for github

[Ailrun]

I have a project using depth-2 subdirectories for packages (like util/validate), and this causes hpack a parse error like the following:

<project path>/util/validate/package.yaml: Error while parsing $.github - expected owner/repo or owner/repo/subdir, but encountered "<owner>/<repo>/util/validate"

Is it possible to support this?

[dpwiz]

The patch is simple, but conflicts with the resolution of #365