policy-node.lua: Second round of cleanup.

- WirePlumber Lua now facilitates Lua libraries/modules, utilize this and create
  modules. Add some tests around this functionality.
- Create policy-hooks.lua containing all the hooks to find-target events
- Create policy-utils.lua module and push all the policy utility functions to it.
- Create common-utils.lua module and push the common utility functions to it.
- Remove all the above functionality from policy-node.lua and clean it up.

src/scripts/lib/common-utils.lua

@@ -0,0 +1,37 @@
+-- WirePlumber
+
+-- Copyright © 2022 Collabora Ltd.
+--    @author Ashok Sidipotu <ashok.sidipotu@collabora.com>
+
+-- SPDX-License-Identifier: MIT
+
+-- Script is a Lua Module of common Lua utility functions
+
+local cutils = {}
+
+function cutils.parseBool (var)
+  return var and (var:lower () == "true" or var == "1")
+end
+
+function cutils.getTargetDirection (properties)
+  local target_direction = nil
+  if properties ["item.node.direction"] == "output" or
+      (properties ["item.node.direction"] == "input" and
+          cutils.parseBool (properties ["stream.capture.sink"])) then
+    target_direction = "input"
+  else
+    target_direction = "output"
+  end
+  return target_direction
+end
+
+function cutils.getDefaultNode (properties, target_direction)
+  local target_media_class =
+  properties ["media.type"] ..
+      (target_direction == "input" and "/Sink" or "/Source")
+  return default_nodes:call ("get-default-node", target_media_class)
+end
+
+default_nodes = Plugin.find ("default-nodes-api")
+
+return cutils