Show revamp

Complete rewrite of 'meteor show', with tests. See commit messages for more details.

This is part of the work on the Meteor tool to get longform package descriptions from README.md files into 'meteor show'. It is safe-contained enough, that it makes sense to submit it for review.

(I won't merge to devel until all the README work is complete -- note that this is being pulled to a staging branch).
this can just be projectContext.layeredCatalog.getPackage, right?  although that prefers local over versioned and this does it the other way around.  ah, and doing it this way ensures that we get stuff like maintainers even if there's a local one.  so this code is correct, but probably deserves a comment explaining why it's different from the normal order