Remove redundant expanduser for download_dir in preparer
Remove redundant expanduser for download_dir in preparer

Since
- download_dir is only set by the download command
- download_dir is
normalized at the beginning of the download command
- path normalization includes expanduser
Therefore expanduser in the preparer is redundant

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