[C++20] [Modules] Enhance better diagnostic for implicit global module and module partitions

Due to an oversight, when users use an unexported declaration from
implicit global module, the diagnostic will show "please #include"
instead of "please import". This patch corrects the behavior.

Also previously, when users use an unexported declarations from module
partitions, the diagnostic message will always show the partition name
no matter if that partition name is visible to the users. Now the users
may only see the partition name if the users are in the same module with
the partition unit.