Reduce generated code

[davidmorgan]

Reduce the generated code to a minimum: just a map from builder name to builder factory, and another map from post process builder name to post process builder factory.

There used to be configuration from yaml files hard coded in the generated script, compute it at runtime instead.

This means

• fewer changes to the script / fewer needs to restart and rebuild
• tests can load the config instead of faking it

but for now don't change restart functionality or tests, keep everything as before.

[github-actions]

PR Health

Changelog Entry ❗

Package	Changed Files
package:build_runner	build_runner/bin/build_runner.dart build_runner/lib/build_runner.dart build_runner/lib/src/bootstrap/build_script_generate.dart build_runner/lib/src/build_plan/apply_builders.dart build_runner/lib/src/build_plan/build_plan.dart build_runner/lib/src/build_plan/builder_factories.dart build_runner/lib/src/build_plan/builder_ordering.dart build_runner/lib/src/build_plan/testing_overrides.dart build_runner/lib/src/build_runner.dart build_runner/lib/src/commands/build_command.dart build_runner/lib/src/commands/daemon_command.dart build_runner/lib/src/commands/run_command.dart build_runner/lib/src/commands/serve_command.dart build_runner/lib/src/commands/test_command.dart build_runner/lib/src/commands/watch_command.dart build_runner/lib/src/internal.dart
package:build_test	build_test/lib/src/test_builder.dart

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