Adds Flutter 3.3 support for flutter plugins library manager

[wagner1343]

Since Flutter 3.3 flutter plugins library manager doesn't updates flutter plugins library correctly (see #6330).

This pull request fixes that by watching both .packages and .dart_tool/package_config.json files. It also uses dart plugin method to extract path map from package_config.json.

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 the [Flutter Style Guide] recently, and have followed its advice.
• 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.

[stevemessick]

Thanks for the contribution! There are a few minor formatting errors, but otherwise this looks good. I'll patch it in and test it once the formatting is done.

[wagner1343]

Fixed those formatting errors, thanks for the review!

[stevemessick]

Thanks again!