Separate Flutter engine & Dart engine cache and pub caches

[ilovelinux]

It could be useful to separate Flutter engine & Dart engine cache and pub and allow to invalidate pub caches as needed.

Actual behavior

1. First run with caching.
   Cache generated: flutter/*
2. Second run, retrieving caches, adding/replacing/updating dependencies.
   The cache has not been updated since it already exists and the update has not been enforced.
3. Third run, no edits.
   Needs to download added/replaced/updated dependencies again since the pub cache hasn't been updated.

Expected behavior

1. First run with caching.
   Two caches were generated: flutter/!.pub-cache with all except .pub-cache folder, and flutter/.pub-cache
   Optionally, we could allow users to define a custom key for pub caches as we do with cache-key input.
2. Second run, retrieving caches and adding/replacing/updating dependencies.
   Enforce pub cache update. If the custom key uses ${{ hashFiles('pubspec.lock') }}, create a new minimal (smaller, without older dependencies) cache.
3. Third run, no edits.
   The cache is valid and ready to use. No need to download new dependencies again.

[bramp]

This may be fixed by #265

[subosito]

Yes, you're correct @bramp. I just updated the v2 and released v2.13.0 for this. Please try @ilovelinux