Explicitly convert depset to list for Bazel compatibility

The flag `incompatible_depset_is_not_iterable` will soon be
enabled in Bazel by default. Implicitly iteration over a depset
will be forbidden.

Closes #70

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=250879220

tools/maven/pom_file.bzl

@@ -148,7 +148,7 @@ def _pom_file(ctx):
     )
 
     formatted_deps = []
-    for dep in _sort_artifacts(mvn_deps, ctx.attr.preferred_group_ids):
+    for dep in _sort_artifacts(mvn_deps.to_list(), ctx.attr.preferred_group_ids):
         parts = dep.split(":")
         if ":".join(parts[0:2]) in ctx.attr.excluded_artifacts:
             continue