refactor(pkg): use dune_section in pkg_rules

[Alizter]

In #14200 I would like to introduce the %{pkg:foo:lib} pfrom more generally outside of package management. In order to do so, it will make sharing both pforms expansions easier if the install section type used is the same.

This change will share the underlying type, whilst preserving the restrictions on "defined" vs "undefined" variables for package management.

For example with both %{pkg-self:...} and %{pkg:foo:...} we don't currently allow for misc, lib_root, libexec_root or share_root. This behaviour is preserved by this change.

[rgrinberg]

If a package section and an opam section aren't available in the same context, why are we making them the same type? Is it just to save a bit of code?

[Alizter]

Yes. I suppose the benefit might be easier to motivate (if at all) after #14200.