'file_lock_dir' setting should fall back if dir doesn't exist

For the 'filesystem' package repository type, the 'file_lock_dir' setting allows for creating the temp lockfile on release in thta directory rather than the package release dir. Unfortunately that means this dir is also expected when doing a rez-build -i to ~/packages for eg.

Rez should simply fall back to the package release dir if the configured file_lock_dir doesn't exist.

Start by tracing the filesystem repository handling for the `file_lock_dir` setting and the `rez-build -i` path used with `~/packages`. Reproduce the case where the configured directory does not exist, then verify that the package release directory is used instead and that the existing behavior remains unchanged when it does exist.

Written by the indexing model from the issue text.