Prepare to enable `explicit-package-dependencies`

[matanlurey]

This flag gates the new "label plugins as dev_dependencies" feature, as well as deprecates two old features:

• https://docs.flutter.dev/release/breaking-changes/flutter-generate-i10n-source
• https://docs.flutter.dev/release/breaking-changes/flutter-plugins-configuration

As of the time of this writing flutter_tools/test/general.shard has 100+ tests that fail with this flag enabled, mostly because they use compute_dev_dependencies transitively, and a fake Pub instance is not hooked up. I have yet to try the other test suites, but likely some fail in commands.shard and integration.shard.

Two options:

1. Opt failing tests out explicitly (turn off the feature), turn the feature on, work through failing tests
2. Fix failing tests before turning on the feature (opt-them in early)

Will chat with @bkonyi and @andrewkolos to figure out what they prefer.

My intuition is (2) leaves us in a better incremental state. Here is an example: #160258.

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TODOs

• Update test/commands.shard/permeable/create_test.dart (and templates/skeleton)
  Potentially also a bug in lib/src/dart/generate_synthetic_packages.dart

[matanlurey]

From talking to @bkonyi and also getting a second opinion from @jmagman, we'll be going with option (2).