rules_r: bazel 0.27 compatibility
rules_r: bazel 0.27 compatibility

Summary:
bazel 0.27 does not allow depsets to be iterable. Most code was already
compatible, but some new code introduced in recent commits was not.

Test Plan:
Manually ran unit tests for this project. I also checked for further
migration problems using bazelisk and there were none.

Reviewers: ysaito

Reviewed By: ysaito

Maniphest Tasks: T12955

Differential Revision: https://phabricator.grailbio.com/D32698

fbshipit-source-id: c38fa8c

grailbio/rules_r,grailbio/rules_r,grailbio/rules_r,grailbio/rules_r,grailbio/rules_r