[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/test.py

@@ -0,0 +1,15 @@
+# pylint: disable=C0111,R0903
+
+"""Test module
+"""
+
+import core.widget
+import core.module
+
+
+class Module(core.module.Module):
+    def __init__(self, config, theme):
+        super().__init__(config=config, theme=theme, widgets=core.widget.Widget("test"))
+
+
+# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4