Analysis Server: implement support for .packages

[sethladd]

Tracking bug. Parent/meta issue is #23372

[pq]

As of 818229c support can be enabled via an option specified in a .dart_analysis_server.yaml file in your homedir. Specifically:

ContextManagerImpl.ENABLE_PACKAGESPEC_SUPPORT: true

[sethladd]

@stereotype441 what do you think, can we close this? How do you feel about this support?

cc @pq

[stereotype441]

We are as complete as we're going to get for 1.12. All the major use cases we're aware of are addressed.

Some rewrite for 1.13 is planned that should tighten up the feature even more, and reduce our risk of unknown bugs, but I don't see any reason to hold up 1.12 waiting for that.

[sethladd]

Thanks for the update!