Replace already_downloaded_path with existence check
Replace already_downloaded_path with existence check

These statements are equivalent, so we exchange them. This will make it
easier to factor the download_dir concerns out of these functions.

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