Simplify dependency generation and improve caching.
Simplify dependency generation and improve caching.

 - Get rid of Config.dep. Store direct dependencies in Symbol.dep
   instead.

 - Use recursive _get_dependent() calls in _get_dependent() to make best
   use of caching.

This roughly doubles the speed of allnoconfig_simpler.py.

ulfalizer/Kconfiglib,ulfalizer/Kconfiglib