Be more careful about ComponentId versus UnitId.

Two big ideas:

    * @--dependency@ takes a ComponentId, not UnitId.
      I used to think it should be a UnitId but it is
      now clear that you want to finger the indefinite
      unit id, which can be uniquely identified with
      a ComponentId

    * When hashing for an InstalledPackageId in
      new-build, we should produce a ComponentId,
      not a UnitId.

While cleaning up the results, for any codepaths which we don't plan on
implementing Backpack (Distribution.Client.Install, I'm looking at you),
just coerce ComponentId into UnitIds as necessary.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>