Set up flutter/core-packages repository #185027

We are spinning up a new repository that will largely follow the structure of flutter/packages, but will conceptually sit between Dart and Flutter, instead of above Flutter (for context see the design doc). The motivation is to have a place for Flutter-team-owned packages that are depended on by flutter/flutter, without creating circular repo dependencies that could cause significant issues in certain update scenarios.

The plan is to have the repository share tooling, by going back to the https://pub.dev/packages/flutter_plugin_tools model where we publish the tools out of flutter/packages, and consume them via pub in flutter/core-packages (as we used to publish from flutter/plugins and use them in flutter/packages, before those repositories were merged). The tooling will need to be made more rigorous about not assuming flutter is available, since flutter/core-packages will, by design, not have Flutter in the CI (except for possibly some specific look-ahead testing task, like we currently do with flutter/flutter running analyze on flutter/packages code).

Roadmap:

• Launch approval for flutter/core-packages creation
• Import vector_math from https://github.com/google/vector_math.dart into flutter/core-packages for initial proof of concept.
• Bring up CI (Cocoon support, pre-submit and post-submit testing, release pipeline with necessary role configuration, etc.) (Enable CI support for flutter/core-packages #185425)
• Fully move vector_math, turning down the google/vector_math.dart repo and publishing from the new location.
• Move flutter/packages packages that should be at this layer:
  • flutter_template_images
  • multicast_dns
  • mustache_template
  • standard_message_codec