config: add config's dir to module search paths

It is convenient for development environments, when the configuration
file and the application sources reside in the same directory.

The same logic was recently implemented for the main script, see #8182.
The same problems appears in context of startup from a configuration
file, so it seems meaningful to adjust module search paths in this case
too.

Part of #8862

NO_DOC=This change is too minor to describe in the documentation issue
       https://github.com/tarantool/doc/issues/3544. I'll work with the
       documentation team regarding details of startup/reload flow and
       we'll determine what should go to the user documentation and what
       shouldn't.