Use rules_pkg repository instead of deprecated pkg files

[dannysullivan]

This repository is currently using a deprecated version of Bazel's package management code. This PR updates all references to @bazel_tools//tools/build_defs/pkg so that they instead use @rules_pkg//, which is maintained.

Note: this PR does not replace rules_docker's custom version of build_tar.py with the one contained in rules_pkg. It should not result in any changed behavior at all.

[dannysullivan]

Closing temporarily pending more testing.

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[smukherj1]

/gcbrun

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dannysullivan, smukherj1

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