Add gazelle plugin to CI and distribution mechanism

[tetromino]

After #400, the gazelle plugin has been cleanly separated out into its own bazel workspace, which will soon finally allow us to mark it stable. But this means:

• we need to change our bazelci config to explicitly build and test it, since bazel build //... no longer includes the plugin;
• we need to add proper distribution rules for it;
• we need to update release instructions, since now we will have two distribution tarballs

[tetromino]

Note: as a followup, I will try to see if we can clean up distro BUILD files to not include distro-only targets. I think this is feasible by updating to a newer version of rules_pkg, but the change is big enough that it would be best done as a separate followup PR.

[achew22]

This LGTM. Obviously gotta fixup CI but conceptually this seems like the right direction. Thanks @tetromino!

[alexeagle]

This module also needs to be added to the Bazel Central Registry in order to use it - @tetromino is that on your list? Want a PR for it?

[tetromino]

@alexeagle - yes, it's my next todo; I just need to generate the tarball

[alexeagle]

I guess it's a wider discussion, but I think we could chat about distribution of the entire repo instead, it's tremendously simpler.