Simplify log filtering

This removes the ability to create nested loggers, and the ability to filter on a regexp over source file path.  In the place of these features, it adds the ability to filter on Go package path.

This PR looks a lot bigger than it is -- the first 4 commits are straight-up ripping code out, and the 5th is a minor refactor.  The actual new functionality is all in the final commit.  I recommend reviewing the individual commits, rather than the overall diff.

Please DO NOT MERGE yet, but I would appreciate review.  I still need to put together PRs to update our code that uses logit.