swarm: move the matrix packages to deploy, where their enable already lives

Which build a service runs is a decision of the host that runs it, so
`*.package` belongs with `enable` rather than in the namespace every hive
must agree on byte for byte.

The `gui` pair was the sharpest case and the reason this starts here:
`gui.enable` moved to `deploy.*` in an earlier slice while `gui.package`
stayed behind, so one web client's two halves sat in two namespaces —
and hive-matrix.nix said so, in a comment directly above the option.
The diff removes a stated inconsistency rather than introducing a rule.

Both old paths keep working through `mkRenamedOptionModule`; both are
settable (neither is `readOnly`), so both get a shim. The `matrixOldPath`
fixture in module-eval.nix now sets eight options through their
pre-rename paths instead of six, so dropping either new shim fails the
eval rather than silently dropping a definition.

Prose swept in the same commit: three comments claimed the package was
swarm-wide or named the pre-rename path, and docs/web-ui/dashboard.md
named `swarm.matrix.gui.package` two lines above
`deploy.matrix.gui.enable` — the same asymmetry this commit exists to
remove, displayed in the docs. A shim keeps the old spelling valid, so
no build, lint or broken-reference search would have found that line.

Refs #3772.

docs/web-ui/dashboard.md

@@ -625,7 +625,7 @@ existing in the page DOM.
 
 A static matrix web client (default `pkgs.fluffychat-web` rebuilt
 with `--base-href /matrix/`, swappable via
-`services.hyperhive.swarm.matrix.gui.package`) served by the hive-gateway
+`services.hyperhive.deploy.matrix.gui.package`) served by the hive-gateway
 nginx container at `/matrix/` when
 `services.hyperhive.deploy.matrix.gui.enable` is on (defaults to
 `matrix.enable`). c0re signals availability via the