build: store Build.modules as a set

Since it's only used to check membership, a set makes more sense. Also
add some documentation to the attribute

mesonbuild/build.py

@@ -275,7 +275,11 @@ class Build:
         self.dependency_overrides: PerMachine[T.Dict[T.Tuple, DependencyOverride]] = PerMachineDefaultable.default(
             environment.is_cross_build(), {}, {})
         self.devenv: T.List[EnvironmentVariables] = []
-        self.modules: T.List[str] = []
+        self.modules: T.Set[str] = set()
+        """Used to track which modules are enabled in all subprojects.
+
+        Needed for tracking whether a modules options needs to be exposed to the user.
+        """
 
     def get_build_targets(self):
         build_targets = OrderedDict()

mesonbuild/interpreter/interpreter.py

@@ -632,8 +632,8 @@ class Interpreter(InterpreterBase, HoldableObject):
             ext_module = NotFoundExtensionModule(real_modname)
         else:
             ext_module = module.initialize(self)
-            assert isinstance(ext_module, (ExtensionModule, NewExtensionModule)), 'for mypy'
+            assert isinstance(ext_module, (ExtensionModule, NewExtensionModule))
-            self.build.modules.append(real_modname)
+            self.build.modules.add(real_modname)
         if ext_module.INFO.added:
             FeatureNew.single_use(f'module {ext_module.INFO.name}', ext_module.INFO.added, self.subproject, location=node)
         if ext_module.INFO.deprecated: