[modules] Separate modules into core & contrib

Also, improve errors when importing a module fails.

Also, add more tests.

modules/core/pasource.py

@@ -0,0 +1,7 @@
+from .__pulseaudio import Module
+
+class Module(Module):
+    def __init__(self, config):
+        super().__init__(config, 'source')
+
+# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4