Rename the add-on from `flamenco3-addon.zip` to `flamenco-addon.zip`.
It still contains the same files as before, and in Blender the name of
the add-on has not changed.
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 installing, Blender will just unzip directly into the addons dir,
so the ZIP has to contain the `flamenco` package directory.
This also makes things simpler, naming-wise. We can offer the addon from
the Manager web interface as `flamenco3-addon.zip`, and still have it
install into the `addons/flamenco` directory.
