Parse go package names

[TG1999]

Follow up of #662
These are problematic because there is no clear documented rule for where a core repo stops and when a path inside a repo starts. The only way will be to use the Go code.

[pombredanne]

The way to get this is to use the same code used by Go:

• use this code and port it to Python https://go.googlesource.com/pkgsite/+/master/internal/source (for instance with a first pass using https://github.com/go-python/gotopy )
  • the source is source.go https://go.googlesource.com/pkgsite/+/master/internal/source/source.go
  • the tests in https://go.googlesource.com/pkgsite/+/master/internal/source/source_test.go show the rules to get the root of a Go package to find its corresponding repo
• for a minimal vendored version see also chore(v2): vendor go/pkgsite/source package for remote URL discovery google/go-licenses#95 and https://github.com/google/go-licenses/tree/master/internal/third_party/pkgsite