This PR introduces a `codeql_pack_rule` that does the heavy lifting
of extracting arch- and common zip files for production dist building.
It also factors out the installer targets for individual packs,
as well as pack groups.
This changes the contract between the internal build system and the pack
definition significantly, which is why an accompanying internal PR is required.
No backwards compatibility layer is provided, as the PR as complex enough as-is.
The individual `codeql_pack` rules are now much simpler,
as they mostly stuff their inputs into a new `_CodeQLPackInfo` provider,
and let the installer and `codeql_pack_group` rules do the heavy lifting.
For working in the external repo with self-contained packs,
the per-pack installer targets are still available.
Internally, we'll only use the new `codeql_pack_group` targets
going forward, both for defining intree-dists and for building
the production zip files.
