Allow importing MMD without xmd["buildrequires"].
When importing modules for offline local builds from local repositories,
the XMD section does not have to be set at all - it gets removed during
the compose and is also MBS specific.
We need to be able to import such MMDs using the `import_mmd` method
in order to make --offline local builds working.
This commit adds new `check_buildrequires` bool kwarg in `import_mmd`
method to disable `xmd["buildrequires"]` checks to fix this.