Disable allow_multiple for per_module_swiftcopt_flag

Bazel doesn't support this yet

swift/internal/build_settings.bzl

@@ -41,7 +41,8 @@ def additional_per_module_swiftcopts(label, provider):
 def _per_module_swiftcopt_flag_impl(ctx):
     # Each item in this list should of the form
     # "<target label>=<comma separated copts>".
-    module_and_copts_list = ctx.build_setting_value
+    # TODO: Remove extra list once allow_multiple is enabled
+    module_and_copts_list = [ctx.build_setting_value]
     value = dict()
     for item in module_and_copts_list:
         if not item:
@@ -72,7 +73,7 @@ def _per_module_swiftcopt_flag_impl(ctx):
 per_module_swiftcopt_flag = rule(
     build_setting = config.string(
         flag = True,
-        allow_multiple = True,
+        # allow_multiple = True,  # TODO: Enable once bazel supports it
     ),
     # TODO(b/186869451): Support adding swiftcopts by module name in addition
     # to the target label.