Allow building module in --offline module with dependencies from local repositories.

There are following changes introduced in this commit:

- The `koji_tag` of module builds imported from the local repositories
  is now in `repofile:///etc/yum.repos.d/some.repo` format to store the
  repository from which the module was imported to local MBS DB.
- The `koji_tag` of fake base module is set to empty `repofile://`
  and in `MockModuleBuilder` the `conf.base_module_repofiles` list
  is used as source for the repositories defining platform. We can't
  simply use single repository, because there might be fedora.repo
  and fedora-update.repo and so on.
- The list of default .repo files for platform are passed using the
  `-r` switch in `build_module_locally` `mbs-manager` command.
- The LocalResolver (subclass of DBResolver) is added which is used
  to resolve the build dependencies when building modules offline
  locally.
- The `MockModuleBuilder` enables the buildrequired modules and
  repositories from which they come in the mock config.

With this commit, it is possible to build testmodule locally
without any external infra.

setup.py

@@ -57,6 +57,7 @@ setup(name='module-build-service',
           'mbs.resolver_backends': [
               'mbs = module_build_service.resolver.MBSResolver:MBSResolver',
               'db = module_build_service.resolver.DBResolver:DBResolver',
+              'local = module_build_service.resolver.LocalResolver:LocalResolver',
           ],
       },
       scripts=['client/mbs-cli'],