fix many file resolution problems which did not expand relative to cwd

This fixes many path resolution problems which were wrong due to chdir() and using `get_path()` instead of `file()`.

It's a work in progress, but each of those in the first commit were failures in my test suite.

Where do you think the tests should go?
I'm hesitating for rubyspec because it takes a while. However there are far too few tests in this area and other rubies would likely benefit from it.