Extension SGFs require the module system to be enabled in order to discover which module defines the extended external type.
This patch ensures the following:
- Associate symbols with their top level module name, and that only top level modules are considered as modules for emitting extension SGFs.
- Ensure we don't drop macro definitions that came from a submodule. To this end look at all defined macros in `PPCalbacks::EndOfMainFile` instead of relying on `PPCallbacks::MacroDefined` being called to detect a macro definition.
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