Cache control of /package/<pkg>/maintainers/ is incorrect

Given that this is dynamic and hopefully cheap-to-generate I think we should prohibit the CDN from caching this endpoint. With the status quo one can be very easily confused by adding a new maintainer and then finding that the change is not reflected in the maintainers list.

Start by locating the handler for /package//maintainers/ and inspect how its response cache headers are set. Make the endpoint prohibit CDN caching, then verify that its response headers prevent stale maintainer lists.

Written by the indexing model from the issue text.