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

// Big Integer Library v. 5.5
// Created 2000, last modified 2013
// Leemon Baird
// www.leemon.com
//
// This file is public domain.   You can use it for any purpose without restriction.
// I do not guarantee that it is correct, so use it at your own risk.  If you use 
// it for something interesting, I'd appreciate hearing about it.  If you find 
// any bugs or make any improvements, I'd appreciate hearing about those too.
// It would also be nice if my name and URL were left in the comments.  But none 
// of that is required.