interpreter: add overload for source_strings_to_files

when only passing strings or Files we only get back Files. This is
useful for the install_* methods

mesonbuild/interpreter/interpreter.py

@@ -2480,6 +2480,10 @@ Try setting b_lundef to false instead.'''.format(self.coredata.options[OptionKey
         if project_root / self.subproject_dir in norm.parents:
             raise InterpreterException(f'Sandbox violation: Tried to grab {inputtype} {norm.name} from a nested subproject.')
 
+
+    @T.overload
+    def source_strings_to_files(self, sources: T.List['mesonlib.FileOrString']) -> T.List['mesonlib.File']: ...
+
     def source_strings_to_files(self, sources: T.List['SourceInputs']) -> T.List['SourceOutputs']:
         """Lower inputs to a list of Targets and Files, replacing any strings.