Support module thinning and renaming Cabal-side.

NB: unlike GHC, Cabal will still try to find a globally consistent choice for
all package names, i.e. this does *not* implement private dependencies.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>
Here, we use the defaultRenaming, because we want the test lib to see all of the exports of the main lib by default. I've added a comment WRT.