Skip doc nodes for packages with no documentable libraries
Check has_documentable_libs before creating compile/doc-all nodes
in build_internal_plan. Packages like base-unix.base, ocaml-config,
and ocaml-options-vanilla have no .mli files and would fail inside
odoc_driver_voodoo. Now they simply don't get doc nodes.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>