Let pip use a local download cache

[joshfriend]

This noticeably speeds up the time it takes to reinstall dependencies.

[jacebrowning]

Why no period prefixing .clean-env but one prefixing .clean-cache?

[joshfriend]

If the . prefixed targets are supposed to be "plumbing commands", .clean-cache is probably one of those commands that people won't need to run specifically or know anything about. clean-env should be exposed so there is a way of rebuilding the virtualenv without clearing the download cache by using clean-all (which would make caching pointless).

[jacebrowning]

@joshfriend Will pip use the cache if there is no internet access?

[joshfriend]

With this configuration, pip will still look at the PyPI index, but doesn't re-download the packages. We could use --no-index and --find-links options to disable the index lookup, but that doesnt sound like a good option.

[jacebrowning]

The best would setup would be don't touch the internet and use the local cache unless a dependency isn't met then download and cache from PyPI. I've been in situations where pip failed simply because it couldn't phone home to PyPI.

[joshfriend]

This could just be rm -rf $(PIP_CACHE_DIR).