Harden PathExclusionsTrait::excludeInProjectRoot() for paths outside of project root.

Problem/Motivation

This method is designed for excluding files in the project root. Hence
every specified path must either be relative OR be absolute inside of
the project root.

So we want to harden it and make it less confusing.

Steps to reproduce

Proposed resolution

Absolute path outside of the project root should trigger an exception and update the Excluders which assumed otherwise.

      if (str_starts_with($path, '/') || str_starts_with($path, 'vfs:')) {
        if (!str_starts_with($path, $project_root)) {
          throw new \LogicException("$path is not inside the project root: $project_root.");
        }
      }
 

Remaining tasks

User interface changes

API changes

Data model changes