Adds imported ZON files to the cache manifest

[MasonRemaley]

fixes #22746

Someone more familiar with the way caching works than me should probably take a quick look at this before it's merged, I only just looked at this part of the codebase for the first time while writing this.

In particular:

1. I'm using addFilePost. I picked this because it's the most similar to what embed file does but doesn't require the file contents, but maybe it's not the right option.

2. I'm unsure about the way I handle the errors that addFilePost can return. Other places in this file seem to have a TODO and just call sema.fail in similar circumstances, so I've done that here, but I feel a little bad proliferating the TODO without understanding it.

[mlugg]

Before merging this, I'm going to see if I can get my PR up and passing instead, since it will tie ZON in with the pipeline more correctly (whereas here it's kinda hacked into Sema). If I can't do that, I'll merge this (barring any CI failures) to fix the immediate major bug.

[mlugg]

Hopefully superseded by #22754.