Assert that target file does not exist
Assert that target file does not exist

_check_download_dir will only return a falsy value if either:

* the provided path does not exist
* the hash does not match - in which case the file is unlinked

so the file cannot exist at either of these points.

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