patch applied (ghc): FIX part of #1372, improvements to the recompilation checker

[Simon Marlow]

Fri Aug  3 03:20:45 PDT 2007  Simon Marlow <simonmar at microsoft.com>
  * FIX part of #1372, improvements to the recompilation checker
  This patch adds a check that each imported module is listed in
  the dependencies from the previous interface.  It catches the
  following cases and forces recompilation:
     - an exposed package has been upgraded
     - we are compiling with different package flags
     - a home module that was shadowing a package module has been removed
     - a new home module has been added that shadows a package module
  
  I haven't yet added the package timestamping as described in #1372.

    M ./compiler/iface/MkIface.lhs -6 +58
    M ./compiler/main/DriverPipeline.hs -6 +6