Speed up native assets target

[dcharkes]

Speeds up the native assets target in the backend by

1. changing other targets gen_dart_plugin_registrant and release_unpack_ios to do async I/O,
2. not reparsing the package config, and
3. not calling dart pub deps --json for 0 or 1 packages (fixed package:native_assets_builder).

• Regression in compilation time due to native asset bundling #134427

           [   +2 ms] native_assets: Starting due to {}
           [   +2 ms] Skipping target: gen_localizations
           [   +1 ms] gen_dart_plugin_registrant: Starting due to {InvalidatedReasonKind.inputChanged: The following inputs have updated contents: /Users/dacoharkes/flt/engine/flutter/examples/hello_world/.dart_tool/package_config_subset}
           [  +33 ms] gen_dart_plugin_registrant: Complete
           [ +107 ms] release_unpack_ios: Complete
           [  +60 ms] Writing native_assets.yaml.
           [   +7 ms] Writing /Users/dacoharkes/flt/engine/flutter/examples/hello_world/.dart_tool/flutter_build/be2692bbfbc0b9a27fcd2422d52354c6/native_assets.yaml done.
           [        ] native_assets: Complete

->

           [   +4 ms] native_assets: Starting due to {}
           [        ] Skipping target: gen_localizations
           [   +1 ms] gen_dart_plugin_registrant: Starting due to {InvalidatedReasonKind.inputChanged: The following inputs have updated contents: /Users/dacoharkes/flt/engine/flutter/examples/hello_world/.dart_tool/package_config_subset}
           [  +31 ms] Writing native_assets.yaml.
           [   +8 ms] Writing /Users/dacoharkes/flt/engine/flutter/examples/hello_world/.dart_tool/flutter_build/f9451a65a465bfab70d004e21d6cc1d6/native_assets.yaml done.
           [   +1 ms] native_assets: Complete

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[christopherfujino]

LGTM

[auto-submit]

auto label is removed for flutter/flutter/134523, due to - The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label.