Reports some dependencies which are not redundant

[thsutton]

I have a package containing a library and a test suite. The library has one exposed-module and a number of other-modules, all in lib/. The test suite uses hs-source-dirs: lib, tests so that it can import the non-exposed modules for testing (but does not include the library itself in the dependencies).

This seems to confuse packunused: it reports the dependencies in the test suite which are included because they are used in the modules imported from the library. Actually removing any of the reported modules results in a compile error.