Fix project directory inference for inspector panel (#6857)
Fixes https://github.com/flutter/devtools/issues/6841, https://github.com/flutter/devtools/issues/6853
Instead of using the top level widget in the widget tree to determine the project directory, we instead use the main isolate's root library.
For google3, we also have extra logic to strip out everything before `/google3`, and to only use top-level directory after `google3` directory (or the top-level directory after `third_party`)
