Change the package base name of the Go code, from
`git.blender.org/flamenco` to `projects.blender.org/studio/flamenco`.
The old location, `git.blender.org`, has no longer been use since the
[migration to Gitea][1]. The new package names now reflect the actual
location where Flamenco is hosted.
[1]: https://code.blender.org/2023/02/new-blender-development-infrastructure/
When the Manager was shutting down while the sleep scheduler was running, it
could cause a null pointer dereference. This is now doubly solved:
- `worker.Identifier()` is now nil-safe, as in, `worker` can be `nil` and
it will still return a sensible string.
- failure to apply the sleep schedule due to the context closing is not
logged as error any more.
