Cross-platform compatibility.

[mktange]

Changed uses of "path" to "path/filepath", in order to make the logic cross-platform compatible.

Added a little bit of extra logic in certain functions of matchers and pkgpath, which makes them work on Windows systems too.

Finally, the tests that are currently failing on Windows have been fixed. To help this, I've added a small extra package osutil, which contains some reusable code-snippets.

Ran all tests on an Unix system, and they still pass.

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This change is