interpreter: Add a holder for StructuredSources

And teach the interpreter how to use it

mesonbuild/interpreter/interpreter.py

@@ -438,6 +438,7 @@ class Interpreter(InterpreterBase, HoldableObject):
             build.InstallDir: OBJ.InstallDirHolder,
             build.IncludeDirs: OBJ.IncludeDirsHolder,
             build.EnvironmentVariables: OBJ.EnvironmentVariablesHolder,
+            build.StructuredSources: OBJ.StructuredSourcesHolder,
             compilers.RunResult: compilerOBJ.TryRunResultHolder,
             dependencies.ExternalLibrary: OBJ.ExternalLibraryHolder,
             coredata.UserFeatureOption: OBJ.FeatureOptionHolder,

mesonbuild/interpreter/interpreterobjects.py

@@ -978,3 +978,9 @@ class GeneratorHolder(ObjectHolder[build.Generator]):
                                             preserve_path_from, extra_args=kwargs['extra_args'])
 
         return gl
+
+
+class StructuredSourcesHolder(ObjectHolder[build.StructuredSources]):
+
+    def __init__(self, sources: build.StructuredSources, interp: 'Interpreter'):
+        super().__init__(sources, interp)