Eliminate zip_file from codebase

[celskeggs]

We're currently maintaining our own version of zip_file.bzl borrowed from an old version of Google's Nomulus repo; they are no longer maintaining this file, because they switched to Bazel, and it was never split out into a separate repository. This means a maintenance load on us to support zip files, as seen in #404.

Luckily, there's already an attempt to support a pkg_zip rule in rules_pkg: bazelbuild/rules_pkg#65. Once this gets merged, we should switch to using that instead. (And if that PR gets abandoned, we should consider whether we have the capacity to help get it finished and merged, because it'd be great to not have to maintain this ourselves.)

We no longer need to use zip_file; #487 is a demonstration of how we can eliminate this dependency.

[celskeggs]

Upstream now supports pkg_zip, at least at a basic level.

[celskeggs]

Unfortunately, the support available in upstream is not yet sufficient for our uses; we need more complex zip file building than they provide.