feat(sterni/acme): add initial plumbing rules

Rules for the Plan 9 plumber (see https://9p.io/sys/doc/plumb.pdf)
allowing quick navigation through depot (and related services) in acme.

- man, urls: some rules that I like taken from /plumb/basic and split
  across individual files.
- tvl: incomplete support for TVL short links and targets:
  - //…
  - cl/…
  - b/…

As mentioned, these are incomplete and I want to implement the remaining
short links and things like git support. I want to continue
experimenting with the plumber, but what I've got so far is already
pretty cool. Eventually, I'll want to implement something equivalent for
Emacs (maybe via Hyperbole) which is probably going to be of general
interest to depot users.

Change-Id: I678f000369db4c7ce30b2aedbc2dbccf3ddef0a9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/13026
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: sterni <sternenseemann@systemli.org>

users/sterni/acme/plumb/default.nix

@@ -0,0 +1,39 @@
+{ pkgs, lib, ... }:
+
+pkgs.stdenvNoCC.mkDerivation {
+  name = "sterni-plumb";
+
+  src = builtins.path {
+    path = ./.;
+    filter = path: _: !(lib.hasSuffix "default.nix" path);
+  };
+
+  dontConfigure = true;
+  dontBuild = true;
+
+  # The write will fail if there's something wrong with the rules,
+  # though it only detects some problems.
+  checkPhase = ''
+    runHook preInstall
+    export NAMESPACE="$(mktemp -d)"
+    9 plumber -f &
+    pid="$!"
+    until [[ -e "$NAMESPACE/plumb" ]]; do
+      sleep 0.1
+    done
+    9 9p write plumb/rules < sterni.plumbing
+    kill "$pid"
+    runHook postInstall
+  '';
+  doCheck = true;
+  checkInputs = [
+    pkgs.plan9port
+  ];
+
+  installPhase = ''
+    runHook preInstall
+    mkdir -p "$out"
+    mv * "$out/"
+    runHook postInstall
+  '';
+}