Absorb pipeline-model-definition symbol-hetero-list.jelly into hetero-list.jelly

There are only two fairly small changes in pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/generator/DirectiveGenerator/symbol-hetero-list.jelly

Right now, it lives as its own file. It would be much nicer if it were possible for it to simply extend the core functionality.

(Specifically, if it were to do that, it would mean that changing the core tag from table to div would just work. Otherwise, it just breaks.)