syntax-highlighter: reference sourcegraph/Packages

[keegancsmith]

I read these outdated references to slimsag/Packages and sent a PR to the wrong repo.

Test Plan: n/a

[Strum355]

Oh I also thought we were still using the fork :clueless: good to know, thanks

[varungandhi-src]

Ugh, sorry about the confusion here. 😖 I made an incorrect comment in sourcegraph/syntect. Looks like we're still using the slimsag fork, as reflected in the Cargo.toml file here.

https://github.com/sourcegraph/sourcegraph/blob/03526e551a5a52f4ce562529afa7e64b43a3dde7/docker-images/syntax-highlighter/Cargo.toml#L12

slimsag/syntect in turn is using slimsag/Packages: https://github.com/slimsag/syntect/blob/bbb71bc0cb5af9488f31a10de5ef409f5b347617/.gitmodules#L4

The main pending items here were:

1. (In Sourcegraph/Packages) Update ST3 branch with Sourcegraph-specific changes Packages#5
2. Bump Packages dependency to point to Sourcegraph fork syntect#9
3. https://github.com/sourcegraph/sourcegraph/issues/34945

I'd started some work on this earlier, but was blocked because of some test failures in Sourcegraph/Packages.

I was planning on making some progress here this iteration; I'll try to get this mess sorted out over the next 1-2 days.