Update installedPkgs when registering.
Update installedPkgs when registering.

This is similar to what we do in Distribution.Simple.Build;
preparatory commit so we can read out the ABI hashes of
our dependencies from installedPkgs.

Signed-off-by: Edward Z. Yang <dbd597f5635f432486c5d365e9bb585b3eaa1853@cs.stanford.edu>

mydaum/cabal,themoritz/cabal,mydaum/cabal,themoritz/cabal,themoritz/cabal