import_module shouldn't be using relative imports of `.config_*`

[pavloo]

Since we can build an absolute path to the module either way, it causes some issues when instead of doing that, we specify package as a second argument (see the doc):

importlib.import_module(
    conf_module,
    conf_package
)

however we should be able just:

importlib.import_module(
    conf_module + conf_package        
)

without any relative imports. what we experiences is that relative import may cause issues in environments where you don't use virtualenv and set PYTHONPATH with python < 3.6