Warn if the source file for a module is ambigious.

[duncan]

For example if we are looking for a module Foo and there are files Foo.hs and Foo.y then we should use Foo.y as we do now, but we should warn about the presence of Foo.hs (and note the fact that we're ignoring it).

Similarly if we find Foo.x and Foo.y then we should warn about the one we're not using. Or arguably that should be a straight error. There's an obvious order relation between Foo.hs and Foo.y but not between Foo.x and Foo.y.