Feature RFC: build-constraints.yaml view commands

[bergmark]

Add some curator commands that reads build-constraints.yaml

• curator view-package <package>
• curator view-maintainer <maintainer>

Format could be similar to what I wanted in #20

This doesn't require any changes to build-constraints.yaml.

Changing the format (explicit comments, issue links, grouping...) would allow us to get more information out of these commands, but I'll stick that in another issue.

[bergmark]

I've moved things like this to https://github.com/bergmark/commenter