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).

* https://github.com/flutter/flutter/issues/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

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] 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].
- [x] All existing and new tests are passing.


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[test-exempt]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
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