Bug when a package is specified by a url to a tarball

[paul-rouse]

This is a small, but serious, error introduced in 99eb520 (on 27 Jan). I'll send a PR in a few minutes to fix it and provide a test case.

Using a reference like this in stack.yaml is now broken:

packages:
- https://hackage.haskell.org/package/half-0.2.2.3/half-0.2.2.3.tar.gz

It produces an error along the lines of "Archive extraction failed. We support tarballs and zip, couldn't handle the following URL, ...", but the underlying problem is that it is trying to use the tarball file name as the directory into which to unpack!

[This is nothing to do with the particular package, which has been chosen here simply because it is a small one for testing purposes.]

[mgsloan]

Good catch, thanks for the PR!