Add allow_single_file in attr.label, deprecate single_file.
Add allow_single_file in attr.label, deprecate single_file.

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MOS_MIGRATED_REVID=126620866

davidzchen/bazel-website,bazelbuild/bazel-website,davidzchen/bazel-website,bazelbuild/bazel-website,davidzchen/bazel-website,davidzchen/bazel-website,bazelbuild/bazel-website