Feature: `base‐path`

base-path

Currently check-spelling has an experimental_path. It should be converted to a more formal base-path to be used to mean something like use this as the working directory for things to check without impacting the source for config.

Tentatively, all paths in excludes and only would be relative to base-path.

Requested

https://github.com/ceph/ceph/pull/46824#issuecomment-1523640048