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/
Blender and FFmpeg were run in the same way, using copy-pasted code. This
is now abstracted away into the CLI runner, which in turn is moved into
its own subpackage.
No functional changes.
