Automated Build Generator

Removed variant and distribution handling in src/**/*.lisp

fixes #2175
fixes #1915

• New variable :disable-ochestration-jobs controls whether orchestration
  jobs are generated.
• Removed all :variant and :distribution[s] variable access. These are
  no longer necessary.

• src/project/classes-spec.lisp (project-spec): added superclass
  `parented-mixin'
  (instantiate version-spec): removed variant stuff
• src/commandline-interface/main.lisp (locate-projects): record the
  distribution which requested a project version
  (load-projects/versioned): install responsible distribution as parent
  of project
  (tag-project-versions): removed; no longer needed
  (configure-buildflow-job): only trigger prepare/finish jobs when they
  exist
  (configure-jobs): allow configuring names of orchestration jobs; only
  generate orchestration jobs when there names are defined
  (configure-distribution): only generate orchestration jobs if
  :disable-ochestration-jobs is not defined
  (main): do not call `tag-project-versions'