Duncan requested that cabal-install not try to put multiple entries in the
package database with duplicate package IDs (foo-0.1) until other tools (in
particular GHC) are able to deal with situation in a better way.  We want to
revert this commit soon, since with this turned on, we basically don't get any
benefit from the package key refactoring (no deliverance from Cabal hell.)

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