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.