Add a flag to allow for several versions of the same package to co-exist in an install plan

[In this Reddit discussion](http://www.reddit.com/r/haskell/comments/132xik/a_longwinded_question_about_the_cabal_problem/c7170a0?context=3), Simon Marlow mentions that it would be nice (and presumably not very difficult) to add a flag to allow several versions of the same package to co-exist in an install plan. Since GHC already versions all symbols, conflicts will be discovered at compilation time.

/cc @kosmikus

No files, tests, or entry points are named. Start by locating the install-plan and dependency-resolution entry points in Cabal, then review the linked Reddit discussion for the intended behavior. Done should include a defined flag and a clear rule for permitting multiple versions of one package while leaving compilation-time conflicts to GHC.

Written by the indexing model from the issue text.