Extract EE-specific code in `lib/gitlab/git_access.rb` to an EE module

The following discussion from !5476 (merged) should be addressed:

• @godfat started a discussion: (+1 comment)

  @rdavila Thank you for porting ee/spec/lib/gitlab/git_access_spec.rb, could you please also port this to an EE module, or maybe also port this to CE? I counted it it's exactly 23 lines as reported.