Accept duplicates when they're all the same.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>
Preliminary support for compiling signature files, using --instantiate-with.

There's no chrome here, but some of the guts for Cabal supporting compiling
signatures.  The key UI is a new --instantiate-with flag for ./Setup (no support
cabal-install side!) which properly modifies the package key, calculates extra
hole dependencies for a package, and ensures an appropriately
translated -sig-of is passed to GHC.  The UI here is supremely user-unfriendly:
the intent is that users will use cabal-install to calculate these parameters
for them.

ToDo: Cabal's inconsistency check in ./Setup needs to be adjusted to be
less zealous.

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

Conflicts:
	Cabal/Distribution/Simple/Configure.hs
	Cabal/Distribution/Simple/LocalBuildInfo.hs
Replace signature-modules with required-signatures/exposed-signatures and PROPERLY check for correct instantiation

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