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/
Instead of sending the current process an interrupt signal, use a dedicated
channel to signal the wish to shut down. The main function responds to that
channel closing by performing the shutdown.
This solves an issue where the Worker would not cleanly shut down on
Windows when `offline` state was requested by the Manager.
