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

{
    "_build_settings": {
        "package": "sat",
        "wrapped_command": "primitivus",
        "dev_repos": {"type": "hg", "url": "https://repos.goffi.org/sat"},
        "icon": "https://repos.goffi.org/sat_media/raw-file/tip/icons_svg/sat.svg",
        "desktop_file": "org.salutatoi.Primitivus.desktop",
        "appdata_file": "_tpl_org.salutatoi.Primitivus.appdata.xml"
    }
}