Move import of modulemd to a separate module

This should make it possible to only import the library only when it's
really needed.

DNF does not work with libmodulemd v2. If we import libmodulemd2 and
then dnf, the program will just hang forever. We only need DNF in
pungi-gather, where libmodulemd is not needed, and also where we do need
libmodulemd, we don't have DNF.

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

tests/helpers.py

@@ -18,7 +18,8 @@ except ImportError:
     import unittest
 
 from pungi.util import get_arch_variant_data
-from pungi import paths, checks, Modulemd
+from pungi import paths, checks
+from pungi.module_util import Modulemd
 
 
 class BaseTestCase(unittest.TestCase):

tests/test_createrepophase.py

@@ -19,7 +19,7 @@ from pungi.phases.createrepo import (CreaterepoPhase,
                                      get_productids_from_scm,
                                      ModulesMetadata)
 from tests.helpers import DummyCompose, PungiTestCase, copy_fixture, touch
-from pungi import Modulemd
+from pungi.module_util import Modulemd
 
 
 class TestCreaterepoPhase(PungiTestCase):

tests/test_gather_source_module.py

@@ -13,7 +13,7 @@ sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
 
 from pungi.phases.gather.sources.source_module import GatherSourceModule
 from tests import helpers
-from pungi import Modulemd
+from pungi.module_util import Modulemd
 
 
 @unittest.skipUnless(Modulemd is not None, "Skipped test, no module support.")

tests/test_initphase.py

@@ -13,7 +13,7 @@ import sys
 
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
 
-from pungi import Modulemd
+from pungi.module_util import Modulemd
 from pungi.phases import init
 from tests.helpers import DummyCompose, PungiTestCase, touch, mk_boom, fake_run_in_threads
 

tests/test_pkgset_common.py

@@ -8,7 +8,7 @@ import mock
 
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
 
-from pungi import Modulemd
+from pungi.module_util import Modulemd
 from pungi.phases.pkgset import common
 from tests import helpers
 

tests/test_pkgset_source_koji.py

@@ -14,7 +14,7 @@ sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
 
 from pungi.phases.pkgset.sources import source_koji
 from tests import helpers
-from pungi import Modulemd
+from pungi.module_util import Modulemd
 
 EVENT_INFO = {'id': 15681980, 'ts': 1460956382.81936}
 TAG_INFO = {
@@ -613,7 +613,7 @@ class MockModule(object):
         return self.path == other.path
 
 
-@mock.patch("pungi.Modulemd.ModuleStream.read_file", new=MockModule)
+@mock.patch("pungi.module_util.Modulemd.ModuleStream.read_file", new=MockModule)
 @unittest.skipIf(Modulemd is None, "Skipping tests, no module support")
 class TestAddModuleToVariant(helpers.PungiTestCase):