refactor(cache): simplify creating / using the `cache` var

[agilgur5]

~NOTE: this is built on top of #414 as it uses the buildStart changes made there. As such, I've marked this PR as "Draft" until #414 is merged.~ Rebased on top and marked as ready for review.

Summary

Use the buildStart hook added in #414 to simplify cache() to cache

Details

• as everything is created in the buildStart hook now (which has RollupContext), we can create the cache there too

  • no need for slightly hacky, lazy creation during transform anymore
  • simplifies it and also standardizes it so it's created the same way as all the other instance vars

• fix: reset cache after each watch cycle

  • previously the cache was never reset, meaning that if anything became dirty in a watch cycle, it would never get reset back
  • I noticed this in the processing of writing watch mode tests in #386
  • in some cases, this would mean that the cache was effectively always dirty during an entire watch mode run, and therefore never used
    • this would be quite inefficient as the FS usage for the cache would just go to waste
  • see that test coverage has now increased as a result!