Fix artifact["exclusions"]

[asinbow]

d98ea59#r60658332

We encounter this bug when using the newest code.

ERROR: An error occurred during the fetch of repository 'unpinned_maven':
   Traceback (most recent call last):
	File "/private/var/tmp/_bazel_haowang/67362b0cd09d93b47f4dadb7394eed77/external/rules_jvm_external/coursier.bzl", line 837, column 38, in _coursier_fetch_impl
		dep_tree = make_coursier_dep_tree(
	File "/private/var/tmp/_bazel_haowang/67362b0cd09d93b47f4dadb7394eed77/external/rules_jvm_external/coursier.bzl", line 770, column 42, in make_coursier_dep_tree
		return deduplicate_and_sort_artifacts(
	File "/private/var/tmp/_bazel_haowang/67362b0cd09d93b47f4dadb7394eed77/external/rules_jvm_external/private/artifact_utilities.bzl", line 49, column 41, in deduplicate_and_sort_artifacts
		if sorted(duplicate_artifact["exclusions"]) == sorted(artifacts_with_exclusions[duplicate_coord]):
Error: key "exclusions" not found in dictionary
ERROR: key "exclusions" not found in dictionary

Not sure whether this is the best solution, please take a look.

[shs96c]

That must have been painful to run into. Thank you for the PR!