[modules] Separate modules into core & contrib

Also, improve errors when importing a module fails.

Also, add more tests.

modules/core/test.py

@@ -0,0 +1,13 @@
+# pylint: disable=C0111,R0903
+
+"""Test module
+"""
+
+import core.widget
+import core.module
+
+class Module(core.module.Module):
+    def __init__(self, config=None):
+        super().__init__(config, core.widget.Widget('test'))
+
+# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4