depmod: create depmod dir independent search function

depmod: create depmod dir independent search function

Prepare to implement external directories support.

The patch splits depmod_modules_search() function to two
functions: depmod_modules_search(), called by the high level with
intention to search all possible modules, and
depmod_module_search_path(), which takes path as a parameter and
scans modules under the path only. Initially it is used to scan
the same depmod->cfg->dirname path only.

Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>