[core] restructure to allow PIP packaging

OK - so I have to admit I *hate* the fact that PIP seems to require a
subdirectory named like the library.

But since the PIP package is something really nifty to have (thanks to
@tony again!!!), I updated the codebase to hopefully conform with what
PIP expects. Testruns so far look promising...

bumblebee_status/modules/core/pasource.py

@@ -0,0 +1,9 @@
+from .pulseaudio import Module
+
+
+class Module(Module):
+    def __init__(self, config, theme):
+        super().__init__(config, theme, "source")
+
+
+# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4