swift-package-manager

[SourceControl] Start sketching a CheckoutManager class.

 - This is intended to abstract out and own the raw checkouts that we need to do
   to perform various operations -- dependency resolution in particular may have
   a need to clone a large number of repositories (some of which are never
   used).

 - My hope is to be able to have a relative abstract interface that simply owns
   the raw checkouts (which I am planning to attempt to take bare clones of, for
   `git`) in such a way that we can clone them in parallel and potentially share
   them across projects.

 - I suspect the SCM subsystem is likely to grow to have a sizeable number of
   pieces, so I carved out a SourceControl module for it.