Move directory requirement download logging out of unpack_file_url
Move directory requirement download logging out of unpack_file_url

One less use of `download_dir` in `unpack_file_url`, which will make it
easier to factor out.

sbidoul/pip,pfmoore/pip,pypa/pip,pfmoore/pip,sbidoul/pip,xavfernandez/pip,pradyunsg/pip,xavfernandez/pip,pypa/pip,pradyunsg/pip,xavfernandez/pip