[flutter_tools] restore pub caching functionality (#70133)

Necessary to work around #70013 (this looks like it will be fixed, but should protect us in the future)

Restores some notion of skipping pub, with differences from the prior implementation:

Off by default, and only used by the flutter command automatic pub get
A single check parameter. Uses the framework version cached per-project instead of a timestamp of the tool, to avoid cases where downloading a prebuilt framework would cause this check to no longer work.