A few cosmetic changes to `acmd cmake depends`:
- do not print legend by default (enable with `--legend`)
- mark Python dependencies with `:py` in printout

Allow the display of Python dependencies between packages. Requires
the additional `packages.py.dot` file.

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.

Ignore a few custom targets in cmake_depends. Also use LCG_97a as an
alternative source for pygraphviz.

Add `acmd cmake depends` sub-command to show package/target
dependencies. This script relies on the `packages.dot` file that needs
to be produced during release building via `cmake --graphviz`.

Relates to ATLINFR-3561.