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>
Can you add a comment saying what this `Bool` stands for? If it controls whether this module renaming is enabled, why not use `Maybe ModRenaming` instead?