Find more uses of imports in patterns

[dzhu]

I had some code, something like the sample below, that was incorrectly triggering the "unused import" warning:

-- A.elm
module A exposing (A(..))

type A =
    A Int

-- B.elm
module B

import A

x =
    \(A.A _) -> 0

This PR fixes that case and several other ones where imported modules are only used in patterns. I tested by updating my code for each case and testing that the warning disappeared when I added the checking code to handle that case (and by adding new tests that failed with the old code but pass with the new code).