Do not download unused sources during command 'sources'
    
    Command 'sources' used to download all files listed in the sources
    file. This is waste of resources, because it is a common packager
    workflow to first update the specfile then get the new source with
    'spectool -g *.spec' and then do 'fedpkg mockbuild'.
    In that situation, the sources file always lists stale files,
    downloading those never achieves anything useful.
    This commit improves the situation by avoiding download of files not
    actually used in the specfile.
    
    The test suite had a strange configuration where specfile did not
    have any sources, but sources file had an entry for 'readme.patch'.
    This led to failures with the new feature. Fixed by adding
    'readme.patch' as a source in the specfile.
    
    Resolves: #559
    Merges: https://pagure.io/rpkg/pull-request/564
    
    Signed-off-by: Otto Urpelainen <oturpe@iki.fi>