FIX 1463 (implement 'ghc-pkg find-module')

FIX 1463 (implement 'ghc-pkg find-module')

- the ticket asks for a module2package lookup in ghc-pkg
  (this would be useful to have in cabal, as well)

- we can now ask which packages expose a module we need,
  eg, when preparing a cabal file or when getting errors
  after package reorganisations:

  $ ./ghc-pkg-inplace find-module Var
  c:/fptools/ghc/driver/package.conf.inplace:
      (ghc-6.9.20071106)

  $ ./ghc-pkg-inplace find-module Data.Sequence
  c:/fptools/ghc/driver/package.conf.inplace:
      containers-0.1

- implemented as a minor variation on listPackages

(as usual, it would be useful if one could combine
multiple queries into one)