Rename `dokkaModuleDirectory`, and mark as internal

[adam-enko]

Follow up for #3876

When Dokka produces 'partial' Modules it stores them in a project-local directory.

How Dokka processes Modules is an implementation detail, so exposing the directory is not useful for users, and could be confusing. It is useful to have a single property for DGP to use though, so it should be kept.

As such, mark baseModulesDirectory as internal to hide it from the DSL.

I propose skipping the Deprecation phase since I think it's very unlikely anyone ever configured this property, even for Dokkatoo users.