Survey code to determine scope of breaking project root assumption

We have a few requests that the Flutter project root directory be separated from the IntelliJ project directory. Actually, I think IntelliJ considers a project to be a collection of modules, independent of their location. That is, the modules do not have to share a common parent directory. One example is #5150.

We need to survey the code base to find where we assume the Flutter project root is the top-level directory in the project.