[meta] Aligning 'flutter pub' and 'dart pub' behavior

[jonasfj]

• Run dart pub get for example/ when running dart pub get in root project.
• Trigger a hook for flutter plugin generation when FLUTTER_ROOT is present, and root package have a flutter dependency.
• Improve HTTP retry logic to recover from broken TCP connections
• Validate content hash for downloaded files (Validate CRC32c if x-goog-hash header is present #2281)
• Support running dart pub -C <folder> where <folder> is used instead of CWD (Add --directory option #2876)
• Refactor flutter pub get to use inheritStdio and fix tests
• Implement metrics collection for better popularity metrics (Analytics #2778)
• Resolve location of PUB_CACHE whether it should be a global cache for Flutter SDK too.
• Ensure flutter help pub <foo> / flutter pub <foo> --help has right output: help on 'flutter pub' does not work as expected flutter/flutter#81011

[jonasfj]

Looks like there is some movement on migrating Flutter away from using a custom PUB_CACHE in flutter/flutter#53833.

[sigurdm]

I believe the cache alignment happened in: flutter/flutter#105343 checking that off.