Try to get the stable modules story right.  Things now work much
better: objects aren't unloaded and reloaded unnecessarily, and
compiling modules from with GHCi works:

	> :! ghc -c A.hs
	> :r
	Compiling A ... compilation IS NOT required (using ./A.o)
	Compiling B ... compilation IS NOT required
	Compiling C ... compilation IS NOT required
	Compiling Main ... compilation IS NOT required

Compiled module must not depend on interpreted modules, but we
currently don't enforce this restriction properly.