[NFC] [C++20] [Modules] Refactor Module::getGlobalModuleFragment and...

[NFC] [C++20] [Modules] Refactor Module::getGlobalModuleFragment and Module::getPrivateModuleFragment

The original implementation of `Module::getGlobalModuleFragment` and
`Module::getPrivateModuleFragment` tried to find the global module
fragment and the private module fragment by comparing strings, which
smells bad. This patch tries to improve this.