Investigate reintroduction of inferpubRoot Directories

[CoderDake]

inferPubRootDirectories was removed in a migration change.

Investigate what regressions this may have caused and if needed, reintroduce the code that calls it

[srawlins]

Closing as resolved: _inferPubRootDirectory has been reintroduced and is actively used in InspectorPreferencesController to detect pub root directories from the main isolate root library URI.

If there is more work to do here, please let us know by filing a new issue with up to date information. Thanks!