Support multiple sources in one variant

With this patch the gather_source option is no longer used. Instead, all
sources are always used. If they return at least some input packages,
then a configured method is used and the returned lists of packages from
all sources are merged.

The method used for gathering can be configured for each variant and
gather source separately.

Additional packages are only added to the comps source.

Each gathering step is logged separately. All the logs are preserved for
later inspection.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

tests/test_pkgset_source_koji.py

@@ -204,7 +204,6 @@ class TestPopulateGlobalPkgset(helpers.PungiTestCase):
                                          pickle_dumps):
         self.compose = helpers.DummyCompose(self.topdir, {
             'gather_method': 'nodeps',
-            'gather_source': 'none',
             'pkgset_koji_tag': 'f25',
             'sigkeys': mock.Mock(),
             'additional_packages': [