Fix merge_if_exists in states.file.serialize

This feature was recently added, but about two weeks before it was
added, this state started importing salt.utils.{yaml,json} as yaml/json.
This makes the yaml.load (or json.load) fail since these functions are
not in the serializer modules.

This fixes merge_if_exists by importing the serializer modules for yaml
and json under a different namespace, allowing the yaml and json modules
to also be loaded.

It also makes the change to using yaml.safe_load, and also adds a
versionadded directive for merge_if_exists.