depmod: cache dependency_symbol list in struct mod

depmod: cache dependency_symbol list in struct mod

The overall goal is to coalesce the accesses to a file that is the
backend of a module. This commit addresses the calls to
kmod_module_get_get_dependency_symbols(). Calling it earlier, while we
are iterating the modules allows us to free the struct kmod of each
module much sooner. We are still not freeing it since there are other
places that must be refactored first.

There's a performance penalty of ~2.5% from previous commit.