Item location relative to package, not 'here'

[theahura]

Previously, find_extra_reqs would concatenate the package_location with the file_location for a library import. This fails when using a virtualenv, because the package_location and the file_location end up sharing paths. This PR changes the file_location to be relative to the package, so that you get an actual file path.

Fixes #75.

[theahura]

Any update on this? It's a pretty low key change, would love to get this merged.

[adamtheturtle]

I do not have availability right now to work on this.

cc @MrMino .

[MrMino]

Some tests for this would be nice.

I still need to reproduce the original issue to make sure this works.