Initial multiple repository support

And lots of code reshuffling which likely should've been done in
separate commits..

package_manager/messages.py

@@ -1,4 +1,4 @@
-from .bpkg import Repository
+from .bpkg.types import Repository
 
 class Message:
     """Superclass for all message sent over pipes."""
@@ -33,7 +33,7 @@ class Success(SubprocMessage):
 class RepositoryResult(SubprocMessage):
     """Sent when an operation returns a repository to be used on the parent process."""
 
-    def __init__(self, repository: Repository):
+    def __init__(self, repository_name: str):
         self.repository = repository
 
 class Aborted(SubprocMessage):