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.

crates/puffin-resolver/tests/resolver.rs

@@ -22,7 +22,7 @@ use puffin_traits::BuildContext;
 struct DummyContext;
 
 impl BuildContext for DummyContext {
-    fn cache(&self) -> Option<&Path> {
+    fn cache(&self) -> &Path {
         panic!("The test should not need to build source distributions")
     }