Combine jimi-c and bcoca's ideas and work on hooking module-utils into PluginLoader.

This version just gets the relevant paths from PluginLoader and then
uses the existing imp.find_plugin() calls in the AnsiballZ code to load
the proper module_utils.

Modify PluginLoader to optionally omit subdirectories (module_utils
needs to operate on top level dirs, not on subdirs because it has
a hierarchical namespace whereas all other plugins use a flat
namespace).

Rename snippet* variables to module_utils*

Add a small number of unittests for recursive_finder

Add a larger number of integration tests to demonstrate that
module_utils is working.

Whitelist module-style shebang in test target library dirs

Prefix module_data variable with b_ to be clear that it holds bytes data

test/sanity/code-smell/shebang.sh

@@ -6,6 +6,7 @@ grep '^#!' -rIn . \
     | grep ':1:' | sed 's/:1:/:/' | grep -v -E \
     -e '^\./lib/ansible/modules/' \
     -e '^\./test/integration/targets/[^/]*/library/[^/]*:#!powershell$' \
+    -e '^\./test/integration/targets/[^/]*/library/[^/]*:#!/usr/bin/python$' \
     -e '^\./test/sanity/validate-modules/validate-modules:#!/usr/bin/env python2$' \
     -e '^\./hacking/cherrypick.py:#!/usr/bin/env python3$' \
     -e ':#!/bin/sh$' \