Abstract away pip url / url pattern support

[tony]

Inspiration: https://wiki.archlinux.org/index.php/VCS_package_guidelines

Other ideas:

• Allow registering regex's on startups (sort of similar to how mimetypes initializes as a singleton. And it also allows adding more types at runtime
• have group regexes catch
• Have vcs detection hint callbacks (and registry). For cases where a vcs is a certain time, a detection callback for having a git/ directory is a sign its a probably git repo. If it has common git-like files, it may be a bare git repo

This allows a directory to be given and a guess of what repo it is

but also its enough to just use the plain old repo object