Rename "blenderpack" module to "bpackage"

Also split code into files by class

bpackage/exceptions.py

@@ -0,0 +1,14 @@
+class BadAddon(Exception):
+    """
+    Raised when something expected to be an addon turns out not to be one after all
+    """
+
+class RepositoryListError(ValueError):
+    """
+    Raised when something is amiss with the repo.json file
+    """
+
+class RepositoryNotFoundError(RepositoryListError):
+    """
+    Raised when looking for a repository which isn't there
+    """