Address Bazel's --incompatible_depset_is_not_iterable in build_defs.bzl

Fixes #529

With this change, the Bazel build is compatible with all flags listed in #529 (comment)

$ bazel build //ijwb:ijwb_bazel_zip --define=ij_product=intellij-latest --incompatible_depset_is_not_iterable --incompatible_depset_is_not_iterable --incompatible_depset_union --incompatible_disable_deprecated_attr_params --incompatible_disable_legacy_cc_provider --incompatible_disallow_legacy_py_provider --incompatible_merge_genfiles_directory --incompatible_new_actions_api --incompatible_use_toolchain_providers_in_java_common
INFO: Build option --define has changed, discarding analysis cache.
WARNING: /usr/local/google/home/jingwen/code/intellij/java/BUILD:177:1: in java_import rule //java:jdk_tools_jar: target '//java:jdk_tools_jar' depends on deprecated target '@local_jdk//:langtools': Don't depend on targets in the JDK workspace; use @bazel_tools//tools/jdk:current_java_runtime instead (see bazelbuild/bazel#5594)
INFO: Analyzed target //ijwb:ijwb_bazel_zip (9 packages loaded, 2036 targets configured).
INFO: Found 1 target...
Target //ijwb:ijwb_bazel_zip up-to-date:
  bazel-bin/ijwb/ijwb_bazel.zip
PiperOrigin-RevId: 249526071

build_defs/build_defs.bzl

@@ -284,7 +284,7 @@ def _plugin_deploy_zip_impl(ctx):
     for target in ctx.attr.srcs:
         data = target[repackaged_files_data]
         input_files = depset(transitive = [input_files, data.files])
-        for f in data.files:
+        for f in data.files.to_list():
             exec_path_to_zip_path[f.path] = output_path(f, data)
 
     args = []