use shared implementation to convert files() strings to File objects

This handles various edge cases:
- checks for sandbox violations just like all other functions
- warn for direntry issues
- check for generated files referred to via strings instead of the
  returned object

(All valid use cases for wanting to sneak around the checks, are made to
work via commit bba588d8b03a9125bf5c4faaad31b70d39242b68.)

mesonbuild/interpreter/interpreter.py

@@ -619,7 +619,7 @@ class Interpreter(InterpreterBase, HoldableObject):
     @typed_pos_args('files', varargs=str)
     @noKwargs
     def func_files(self, node: mparser.FunctionNode, args: T.Tuple[T.List[str]], kwargs: 'TYPE_kwargs') -> T.List[mesonlib.File]:
-        return [mesonlib.File.from_source_file(self.environment.source_dir, self.subdir, fname) for fname in args[0]]
+        return self.source_strings_to_files(args[0])
 
     # Used by declare_dependency() and pkgconfig.generate()
     def extract_variables(self, kwargs, argname='variables', list_new=False, dict_new=False):