Fix package root resolution after build_runner 2.14.0 AOT changes

[Carapacik]

What does this change?

This fixes package root resolution in flutter_gen_runner after the build_runner 2.14.0 change that runs build / watch through AOT by default.

Previously, FlutterGenBuilder relied on Isolate.packageConfigSync! as the only source for package roots. With the newer AOT build-script flow, the runtime isolate package config can be null or no longer represent the active build graph, which may crash the builder or fail to resolve the target package root, especially on Windows.

The package root is now resolved in this order:

1. Use BuildStep.packageConfig as the primary source for the active build graph.
2. Fall back to the runtime isolate package config when available.
3. Fall back to discovering the package from .dart_tool/package_config.json or a matching pubspec.yaml under the current working directory.

This also removes the eager static runtime package config load, avoiding the null assertion crash from Isolate.packageConfigSync!.

Fixes #749 🎯

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

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• Appropriate docs were updated (if necessary)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.37%. Comparing base (59626d9) to head (15c4103).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #750   +/-   ##
=======================================
  Coverage   97.37%   97.37%           
=======================================
  Files          24       24           
  Lines         991      991           
=======================================
  Hits          965      965           
  Misses         26       26           

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[AlexV525]

Thanks for the fix!