Fix #11458, pt.1: invoking jam from source dir does not work.

* Apply change suggested by Ingo: replace LocalDepends with Depends.
Fix #11458, pt.2: invoking jam from source dir does not work.

* Adjust package_repo create command to check if each given package
  exists before adding it to the repository writer and fail with
  an appropriate message when it does not.
Fix #11458, pt.3: invoking jam from source dir does not work.

* Adjust build_haiku_repository script to invoke package_repo without
  -C <dir>, as that is not needed and has the unintended side-effect
  of rendering relative package paths non-functional.