In order to improve interaction with the pubroot directories. The pubRoot directory interface is being changed to promote adding and removing directories. This will make it so that IDEs and Devtools can't overwrite each other's changes to the pub root directories if they call setPubRootDirectories at the same time.

Fixes #105730

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.