Every Haskell compilation in a package should depend on the
package-data.mk file for that package.  Then, if a package's version
number is bumped, we will recompile all the modules in the package so
that their interface files get the new version number.

This should mean that a simple 'make' will succeed in a couple of
scenarios where it didn't before:

  1. re-configuring bumps the version number of GHC, so everything
     in the ghc package (stages 1 & 2) must be recompiled.

  2. bumping the version of one of the boot packages does not require
     a make clean any more.

I have tested (1) but not (2).

LAX_DEPENDENCIES=YES disabled these new dependencies too.