PyUtils.cmake_depends: further tweaks for batch processing

A few more tweak to acmd.py cmake depends:

• Only create sub-graphs when more than one package/target was specified on the command line. This helps with digesting the files by external libraries like vis.js that do not support sub-graphs.
• Restrict the --long option to txt output as it wasn't working for Python dependencies in the DOT graph anyway. Also fix error handling.