dump-decls: fix "Ambiguous module name"-error

Fixes errors of the following kind, which happen when dump-decls is run on a package that contains a module name that clashes with that of another package.

```
dump-decls: <no location info>: error:
    Ambiguous module name `System.Console.ANSI.Types':
      it was found in multiple packages:
      ansi-terminal-0.11.4 ansi-terminal-types-0.11.5
```