Refactor hpcmain

This PR does mainly two things:

• Simplify the helpList function by explicitly passing the plugins to the section function, instead of passing the names of the plugins which then have to be looked up.
• Split the big dispatch function into three smaller functions on the toplevel: dispatch, dispatchOnPlugin and dispatchOnResponseFiles

This is all in preparation of the simplification of the command line parsing logic which is distributed all over the codebase.