Add a dependency cache.
Add a dependency cache.

This is a file called ~/.pip-tools/dependencies.pickle, which contains
dependency information on every package version that has been
downloaded, unpacked, built and read requires.txt from, so this does not
have to be done again (for that version).

Also, this cached dependencies between runs.

suutari/prequ,suutari-ai/prequ,suutari/prequ