Filter on caller's PC rather than logger name

Overrides can be specified in one of three formats, in order of increasing
specificity:

  * "path/to/pkg" filters based on the Go package path of the logger.
  * "path/to/pkg/filename.go" filters based on the filename of the logger.
  * "path/to/pkg/filename.go:123" filters based on filename and line number of
    the logger.

When filtering a log record, the most specific override wins.