Fix most linter warnings reported by 'staticcheck'. This doesn't fix all
of them, some unused functions are still there, and some generated code
also still triggers some warnings. Most issues are fixed, though.
No functional changes, except for the captialisation of some error
messages.
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/
Move the UPnP/SSDP Manager autodiscovery code into from `main.go` into the
`worker` package. This also means changing the error handling a bit, as
only the `main.go` file is allowed to do `log.Fatal()`.
