Shadowing warning improvement

[yav]

When we check for shadowing, we should only report a warning if the shadowed name is different than the name being defined.
Usually, that would not be an issue as we have no recursive modules, but it can happen quite easily with submodules:

module A where

submodule B where
  z = 0x2

import submodule B

At the moment this results in:

[warning] at test.cry:4:3--4:4
    This binding for `z` shadows the existing binding at
    test.cry:4:3--4:4

[yav]

Should be fixed after the merge of the nested modules PR