When chasing explicit imports, we now look for the module in both the
home package and external packages, and complain if it is found in
both:

Chasing modules from: Overlap
Cannot import `List':
  it was found in both overlap/List.hs and package haskell98-1.0.
  Possible fix: -ignore-package haskell98-1.0
  (imported from overlap/Overlap.hs)

Previously the home module would have been ignored in favour of the
package module.  It means we're doing a bit more searching, but the
behaviour is less obscure.