Add a way for to provide the `package_config.json` location for a test target

[kenzieschmoll]

This is so that VM service clients connected to a test run can perform an evaluation to get the package_config.json file location for the test.

This will require updating every tool that generates a test entrypoint to include this for evaluation

• package:test
• flutter tools
• blaze (g3)
• build_runner

Action steps:

• Prepare the changes in package:test (or flutter tools may be easier to verify) - Add package config URI into VM test entrypoints #2245
• @kenzieschmoll will verify the method returning the package config file path can be evaluated from a VM service client (DevTools) - Lookup the package config location for Dart and Flutter test targets flutter/devtools#7941
• Changes will be landed across the different test entrypoints
  • Write the package config location to the test bootstrap. flutter/flutter#150440
  • Add package config URI into VM test entrypoints #2245

@natebosch @jakemac53 @bkonyi

[kenzieschmoll]

The changes have landed across the Dart SDK, flutter_tools and DevTools. So this is now resolved.