Hacking on improved VCS support

This patch covers a new VCS abstraction with corresponding tests. It
also re-implements the get command in terms of the VCS and adds unit
tests (including optional network tests).

This does slightly change the behaviour of the get command: instead of
looking for which VCSs are available on the system and then choosing
which repo to try and get, we instead pick a preferred repo and then we
look for the corresponding VCS and fail if it's not available. In
practice this difference does not matter because we don't have multiple
repos for the same package using different VCSs.