Introduce allowedRequisites feature
Fix manual build
allowedRequisites: Drop stdenv mention

I don't think it's a good idea to use allowedRequisites for stdenv, so
don't mention it here.
Add disallowedReferences / disallowedRequisites

For the "stdenv accidentally referring to bootstrap-tools", it seems
easier to specify the path that we don't want to depend on, e.g.

  disallowedRequisites = [ bootstrapTools ];