Make cache non-optional in most crates (#293)
This PR makes the cache non-optional in most of Puffin, which simplifies
the code, allows us to reuse the cache within a single command (even
with `--no-cache`), and also allows us to use the cache for disk storage
across an invocation.
I left the cache as optional for the `Virtualenv` and `InterpreterInfo`
abstractions, since those are generic enough that it seems nice to have
a non-cached version, but it's kind of arbitrary.
