Don't re-export top-level modules

In https://reviews.llvm.org/D119036 we fixed some of the infrastructure by
removing the textual keyword.

The underlying issue of PR50592 was that clang can re-export only submodules but
under some conditions we needed to re-export the standalone module std_config
via std. This patch provides a better fix to the symptom D119036 fixed.

Differential revision: https://reviews.llvm.org/D142805