osbuild: don't use `util.get_repo_urls()`
    
    Don't use `util.get_repo_urls()` to resolve provided repositories, but
    implement osbuild-specific variant of the function named
    `_get_repo_urls(). The reason is that the function from `utils`
    transforms repositories defined as dicts to strings, which is
    undesired for osbuild. The requirement for osbuild is to preserve the
    dict as is, just to resolve the string in `baseurl` to the actual
    repository URL.
    
    Add a unit test covering the newly added function. It is inspired by a
    similar test from `test_util.py`.
    
    Signed-off-by: Tomáš Hozza <thozza@redhat.com>