Find outside-package test dependencies too.

I added a call to `go list` using `XTestImports`, and a test for outside-package test dependencies. To do that, I had to add a `pkg_name` field to the `file` struct, and the matching values to all tests.

I also added a deferred removal of temporary test directories.

Fixes issue #11.
Ok, I see now that it deletes a temporary GOPATH rather than something real.

Perhaps `tempGopath` would be a better var name (or a comment). Or it's fine since the code is right there, I just didn't see it in the PR at first before expanding.