Turn //tools/build_rules into a package.
Turn //tools/build_rules into a package.

This is in preparation for another change that will add a new helper script
to the tools/build_rules directory, and such script requires a BUILD rule
of its own.

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MOS_MIGRATED_REVID=114898083

bazelbuild/rules_rust,bazelbuild/rules_rust,bazelbuild/rules_rust,bazelbuild/rules_rust,bazelbuild/rules_rust