pip-install-simple caching

I have dodgy internet due to where I live.
When I rebuild, occasionally it fails because PyPI.org is not accessible, however I have a previously fetched a perfectly acceptable copy of the package very recently, and would like to keep building with the same version, at least as a fallback-with-warning if a fresh copy couldnt be fetched.

The issue does not name a file, test, or entry point. Start by locating the pip-install-simple implementation and tracing how packages fetched from PyPI are handled during rebuilds. Done means a recently fetched package can be reused when PyPI is unavailable, with a warning when the fallback is used.

Written by the indexing model from the issue text.