Abstract the fetching mechanism

[Araq]

The fetcher is the component that does the following:

• first rewrite the URL
• then depending on the URL, use git or hg or nimgrab or curl or wget ...

[elcritch]

Overloading from the URL is bit tricky in general as outside of known sites like github or gitlab it can be difficult to determine if it's git vs hg.

Mercurial projects don't seem to include a .hg in their URLs from what searching I've seen.

So worse case I think you'd need a way to for users to manually specify the type in case the default URL parsing fails to detect the resource type.

[Araq]

Pip uses the git+https://... syntax iirc.