Revert "[Parse] Split incremental-extensions" (#66281)

This reverts commit c2fb112021529c635cccd8bb9d846b2c64fc291d, which
breaks:
```
lldb-api.commands/expression/diagnostics.TestExprDiagnostics.py
lldb-api.lang/objc/modules.TestObjCModules.py
lldb-api.lang/objc/modules-incomplete.TestIncompleteModules.py
lldb-api.lang/objc/modules-non-objc-target.TestObjCModulesNonObjCTarget.py
lldb-api.lang/objc/modules-objc-property.TestModulesObjCProperty.py
```