Initial multiple repository support

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

package_manager/bpkg/exceptions.py

@@ -9,3 +9,6 @@ class DownloadException(BpkgException):
 
 class BadRepositoryException(BpkgException):
     """Raised when there is an error while reading or manipulating a repository"""
+
+class PackageException(BpkgException):
+    """Raised when there is an error while manipulating a package"""