Partial mirroring

The current version of pep381run downloads all of the packages available.

It would be nice to be able to perform selective/partial mirroring, ie, to just download a subset of all the packages available, where those packages may be specified either as a command line argument or with a requirements file.

This would be very useful for those who do not want a complete mirror, but rather a backup repository for the packages that they usually need for their software, which can be used when no internet connection is available. This approach would help spare disk space and bandwidth by avoiding downloading and storing unneeded packages.

I'm working on this issue. See my branch.