Pass buildrequired modules built against all compatible base module streams to MMDResolver.

Imagine we have "platform:f29.0.0" and "platform:f29.1.0" base modules.
We also have "DBI" module we want to build agaisnt "platform:f29.1.0".
This "DBI" module depends on "perl" module which is only build against
"platform:f29.0.0".

Currently, DBI build would fail to resolve the dependencies, because
it wouldn't find "perl" module, because it is built against different
platform stream.

This PR changes the MSE code to include buildrequired module builds built
against all the compatible platform streams.

It does so by introducing following changes:

- MSE code uses new get_base_module_mmds() method to find out all the
  compatible platform modules. This needed new methods in DBResolver
  and MBSResolver.
- For each buildrequired module defined by name:stream, the MSE code then
  finds particular NSVC built against each compatible platform module.

Side effect of these code changes is that every module now must buildrequire
some base module.

tests/test_resolver/test_db.py

@@ -22,11 +22,14 @@
 
 import os
 
+from datetime import datetime
 from mock import patch, PropertyMock
 import pytest
 
 import module_build_service.resolver as mbs_resolver
 from module_build_service import app, db, models, glib, utils, Modulemd
+from module_build_service.utils import import_mmd, load_mmd
+from module_build_service.models import ModuleBuild
 import tests
 
 
@@ -38,6 +41,54 @@ class TestDBModule:
     def setup_method(self):
         tests.reuse_component_init_data()
 
+    def test_get_buildrequired_modulemds(self):
+        mmd = load_mmd(os.path.join(base_dir, 'staged_data', 'platform.yaml'), True)
+        mmd.set_stream('f30.1.3')
+        import_mmd(db.session, mmd)
+        platform_f300103 = ModuleBuild.query.filter_by(stream='f30.1.3').one()
+        mmd.set_name("testmodule")
+        mmd.set_stream("master")
+        mmd.set_version(20170109091357)
+        mmd.set_context("123")
+        build = ModuleBuild(
+            name='testmodule',
+            stream='master',
+            version=20170109091357,
+            state=5,
+            build_context='dd4de1c346dcf09ce77d38cd4e75094ec1c08ec3',
+            runtime_context='ec4de1c346dcf09ce77d38cd4e75094ec1c08ef7',
+            context='7c29193d',
+            koji_tag='module-testmodule-master-20170109091357-7c29193d',
+            scmurl='git://pkgs.stg.fedoraproject.org/modules/testmodule.git?#ff1ea79',
+            batch=3,
+            owner='Dr. Pepper',
+            time_submitted=datetime(2018, 11, 15, 16, 8, 18),
+            time_modified=datetime(2018, 11, 15, 16, 19, 35),
+            rebuild_strategy='changed-and-after',
+            modulemd=mmd.dumps()
+        )
+        build.buildrequires.append(platform_f300103)
+        db.session.add(build)
+        db.session.commit()
+
+        resolver = mbs_resolver.GenericResolver.create(tests.conf, backend='db')
+        result = resolver.get_buildrequired_modulemds(
+            "testmodule", "master", platform_f300103.mmd().dup_nsvc())
+        nsvcs = set([m.dup_nsvc() for m in result])
+        assert nsvcs == set(['testmodule:master:20170109091357:123'])
+
+    @pytest.mark.parametrize('stream_versions', [False, True])
+    def test_get_module_modulemds_stream_versions(self, stream_versions):
+        tests.init_data(1, multiple_stream_versions=True)
+        resolver = mbs_resolver.GenericResolver.create(tests.conf, backend='db')
+        result = resolver.get_module_modulemds(
+            "platform", "f29.1.0", stream_version_lte=stream_versions)
+        nsvcs = set([mmd.dup_nsvc() for mmd in result])
+        if stream_versions:
+            assert nsvcs == set(['platform:f29.1.0:3:00000000', 'platform:f29.0.0:3:00000000'])
+        else:
+            assert nsvcs == set(['platform:f29.1.0:3:00000000'])
+
     @pytest.mark.parametrize('empty_buildrequires', [False, True])
     def test_get_module_build_dependencies(self, empty_buildrequires):
         """