Make the get_sources shell script support flat layout
    
    The idea of this commit is to make get_sources.sh support both the
    'traditional' git layout used on dist-git for CentOS Linux where the
    spec files are stored in a SPECS folder and the patches in a SOURCES
    folder with the sha of the tarball being in a '.<pkg_name>.metadata'
    file as well as the 'flat' git layout that have adopted Fedora as well
    as CentOS-Stream (9+) where the spec files and patches are all stored
    at the top level directory of the repository and the sha of the tarball
    is present in a 'sources' file.
    
    This commit re-uses code from the fedpkg-minimal project which is
    license under the GPL (so the License field of the spec file for
    centpkg-minimal may need to be adjusted to MIT and GPL).
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>