This has the same motivation as fetchFromGitHub/fetchFromGitLab --
it's cheaper to download a tarball of a single revision than it is to
download a whole history.
I could have gone with domain/group/repo, like fetchFromGitLab, but it
would have made implementation more difficult, and this syntax means
it's a drop-in replacement for fetchgit, so I decided it wasn't worth
it.
In my opinion Functions should only contain pure functions. These are
both meant to provide derivations so I put them under Builders. Don't
know exactly *where* to put them so "special" it is...
