[C++20][Modules][8/8] Amend module visibility rules for partitions.

Implementation partitions bring two extra cases where we have
visibility of module-private data.

1) When we import a module implementation partition.
2) When a partition implementation imports the primary module intertace.

We maintain a record of direct imports into the current module since
partition decls from direct imports (but not trasitive ones) are visible.

The rules on decl-reachability are much more relaxed (with the standard
giving permission for an implementation to load dependent modules and for
the decls there to be reachable, but not visible).

Differential Revision: https://reviews.llvm.org/D118599