flatpak: update build-manifest to retrieve dependencies without initial download phase:
- dependencies are now entirely retrieved from the installation, and requirement are build
from there. The source package is currently always used, and it is now possible to use
PYTHON_DEP_PREINSTALL to add extra packages or use a specific version of a package.
- fixed insertion of `release_elt` for dev version at the right location.
- `branch` is not set anymore for stable versions.
- libxslt is not added anymore as it is part of freedesktop runtime
- flathub share-modules is now used for Python 2