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.