[Distutils] Overriding dependency versions

[Daniel Holth]

> I don't think it should be up to the tool to decide that a dependency is wrong.
>
> IMO, the tool should satisfy the declared dependencies as well as possible,
> report conflicts, and give the user a way to decide a conflict.

In this case we don't expect a conflict, but are unfortunate enough to
be using a dependency that pins some other project. The tool would
ideally warn us that there was a newer version of some pinned
dependency on an interior node of our dependency graph and let us do
something about it.