If a dependency contains a symlink named 'vendor', and that symlink points to
a directory, delete the symlink. Keep the directory around - we can tackle
that part when we get to actually pruning the vendored dependencies.

Also, add some test helpers for testing filesystem state changes.

Relative symlinks on windows confuse filepath.Walk. This is Go issue 17540,
golang/go#17540. While waiting for that to get fixed,
just use absolute symlinks in windows tests.

On Windows, we're unable to distinguish between symlinks and junctions. Deleting
junctions is dangerous and appears to be able to delete the underlying folder,
not just the junction that's pointing to a folder. That's unacceptably
dangerous: a malicious repo could contain a junction named 'vendor' pointing at
'C:\\' and we'd delete all the user's data.

It's too hard to create junctions with permissions that let filepath.Walk
process them.