The purpose of this PR is to make it so that when the user runs 'flutter', if they have a .pub-cache directory in their flutter root, we use that instead of the default location for the pub cache. Otherwise, it should act as before.

The eventual goal is to support a pre-populated flutter .zip/.tar.gz file that has everything the developer needs in one bundle. In order for that to actually work, we need to have the pub cache be self-contained, and not in the user's home dir.

Another advantage of this is that if you have multiple flutter repos that you're switching between, then the versions in the pub cache will remain static when you switch between them.