Force-reload all sub-modules of packages recursively.

See sopel-irc#1056.

sopel-irl#1314 used recursion to reload nested modules. However, in Python 2 (maybe python 3 as well), this approach leads to odd behavior with callables being unloaded/reloaded twice. The loop detects my_module.my_callable and my_callable as two separate modules to reload. The first time it detects it, the rule is added to bot._callables as a string.
I combined patched dgw branch 1056-no-dupes onto 1056-fix-with-recursion, to provide a more optimal solution to issue sopel-irc#1056.