Refactor filters

This includes a few changes initiated by logsearch/logsearch-boshrelease#58...

 * this creates a `defaults` directory which has the previous filters at the same priority levels
 * all the `src` filters lose their priority prefix (which makes testing easier and avoid the priority dependency on the inclusion name)
 * this cleans up `build.sh` a tiny bit for verbosity and allows it to build both `src` (for testing, default - same as before) and also `defaults` (for boshrelease packaging and artifact)
 * this cleans up `package.sh` to make it work with `target` or a different built directory