Figure out what to do with "reproducible attributes" returned by repo rules

Repo rules can return a set of attribute values to make themselves reproducible. We currently print them to stderr, but that's often useless in Bzlmod because the user who sees the info message isn't going to do anything about it -- they're not about to go into the module extension and change the calls. We should figure out how to use this information.

Start by tracing where repo rules return reproducible attribute values and where those values are currently printed to stderr, with particular attention to Bzlmod usage. The work is done when there is an agreed, useful way to use this information instead of only showing an info message.

Written by the indexing model from the issue text.