transitive imports

[Ross Paterson]

Given these modules:

	module A(Stuff(Foo, Bar)) where
	data Stuff = Foo | Bar

	module B(Stuff(..)) where
	import A

	module C where
	import B

loading C gives "INTERNAL ERROR: importEntity"

Changing the import in C gives different internal errors.  The particular
form of the exports in A and B and the import in B seems necessary to
trigger the bug.