Fix project directory inference for inspector panel (flutter#6857)

Fixes flutter#6841, flutter#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`)

packages/devtools_app/release_notes/NEXT_RELEASE_NOTES.md

@@ -23,9 +23,6 @@ To learn more about DevTools, check out the
 ## Performance updates
 
 * Changed raster layer preview background to a checkerboard. - [#6827](https://github.com/flutter/devtools/pull/6827)
-
-## CPU profiler updates
-
 TODO: Remove this section if there are not any general updates.
 
 ## Memory updates