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

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