Own implementation of conda channel mirroring

This MR implements a command to perform conda channel mirroring followed by re-indexing.

It should replace the use of conda-mirror in your server.

Mitigates the remaining issue with #41 (closed).

conda/meta.yaml

@@ -62,6 +62,7 @@ test:
     - bdt dumpsphinx https://docs.python.org/3/objects.inv > /dev/null
     - bdt create --help
     - bdt build --help
+    - bdt mirror --help
     - bdt rebuild --help
     - bdt test --help
     - bdt caupdate --help