i3status-rust, docker-compose: each module declares the packages it needs (issue 628)

i3status-rust left pacman-contrib to the pacman module because two modules declaring one package was once refused; ADR 0303 allows it, so the bar declares what its update block runs. docker-compose's README said the package goes with the module, which the node-engine never does.

modules/i3status-rust/README.md

@@ -2,7 +2,7 @@
 
 The bars as a module (novox/hq ADR 0208, research 026/05).
 
-- Installs `i3status-rust`. The update block's `checkupdates` comes from `pacman-contrib`, which the `pacman` module installs on every node; two modules declaring one package is refused at composition.
+- Installs `i3status-rust`, and `pacman-contrib` for the update block's `checkupdates`. The `pacman` module declares `pacman-contrib` too; several modules may declare one package (novox/hq ADR 0303), so each declares what it needs.
   Claims the mesh's `node-bar` seat (no verbs yet, ADR 0208 §2) and renders the blocks other modules
   contribute to it (ADR 0255, below). Requires `x11-display` on its own
   machine: its bars are i3bar's.

modules/i3status-rust/module.json

@@ -35,6 +35,11 @@
       "type": "package",
       "package": "i3status-rust"
     },
+    {
+      "id": "updates",
+      "type": "package",
+      "package": "pacman-contrib"
+    },
     {
       "id": "configuration-dir",
       "type": "directory",