Introduce new names of project receiver roles

This is the first step for renaming the following receiver roles:

- `watcher` to `project_watcher`
- `source_watcher` to `source_project_watcher`
- `target_watcher` to `target_project_watcher`

Co-authored-by: Saray Cabrera Padrón <scabrerapadron@suse.de>