Several improvements for `acmd.py cmake depends`:
- Dependencies of an external library where not displayed correctly.
  Also warn the user in case the `--external` option is not used.
- Improve filtering of custom targets. Instead of using a hard-coded
  list, parse the graphiz legend and use the target types for filtering.
- Add an `--all` option to disable any filtering of custom targets.
- Add a testing script. It's not run as a unit test because the output
  of the script would of course change from nightly to nightly. But it
  can be used as a manual test script to find regresssions.