melpaBuild: allow nix unstable version format

Before, an unstable version of melpaBuild has to be in MELPA format,
which has some disadvantages:
- existing update scripts cannot be reused[1]
- it causes inconsistence.

This patch fixes that by allowing an unstable version to be in Nix
format.

We heuristically detect if it is an unstable version in Nix format.
If so, we convert it to MELPA format.

This does not bring evaluation performance overhead.  Here is my
benchmark result using hyperfine-1.18.0 and nix-2.18.1.

Without this patch:

Benchmark 1: NIXPKGS_ALLOW_BROKEN=1 nix eval --include nixpkgs=$PWD --file . emacs.pkgs.melpaPackages --apply 'pkgSet: map (drv: drv.drvPath) (builtins.filter (p: p.type or null == "derivation") (builtins.attrValues pkgSet))' --no-eval-cache >/dev/null
  Time (mean ± σ):      9.126 s ±  0.048 s    [User: 6.531 s, System: 0.997 s]
  Range (min … max):    9.056 s …  9.186 s    10 runs

With this patch applied:

Benchmark 1: NIXPKGS_ALLOW_BROKEN=1 nix eval --include nixpkgs=$PWD --file . emacs.pkgs.melpaPackages --apply 'pkgSet: map (drv: drv.drvPath) (builtins.filter (p: p.type or null == "derivation") (builtins.attrValues pkgSet))' --no-eval-cache >/dev/null
  Time (mean ± σ):      9.124 s ±  0.057 s    [User: 6.522 s, System: 1.011 s]
  Range (min … max):    9.040 s …  9.220 s    10 runs

[1]: 177c9eddad/pkgs/applications/editors/emacs/elisp-packages/manual-packages/lsp-bridge/default.nix (L93-L108)

pkgs/build-support/emacs/melpa.nix

@@ -38,6 +38,11 @@ in
     Default: pname
   */
 , ename ? pname
+  /*
+    version: Either a stable version such as "1.2" or an unstable version.
+    An unstable version can use either Nix format (preferred) such as
+    "1.2-unstable-2024-06-01" or MELPA format such as "20240601.1230".
+  */
 , version
   /*
     commit: Optional package history commit.
@@ -71,6 +76,19 @@ genericBuild ({
 
   inherit packageBuild commit ename recipe;
 
+  melpaVersion =
+    let
+      parsed = lib.flip builtins.match version
+        # match <version>-unstable-YYYY-MM-DD format
+        "^.*-unstable-([[:digit:]]{4})-([[:digit:]]{2})-([[:digit:]]{2})$";
+      unstableVersionInNixFormat = parsed != null; # heuristics
+      date = builtins.concatStringsSep "" parsed;
+      time = "0"; # unstable version in nix format lacks this info
+    in
+    if unstableVersionInNixFormat
+    then date + "." + time
+    else version;
+
   preUnpack = ''
     mkdir -p "$NIX_BUILD_TOP/recipes"
     if [ -n "$recipe" ]; then
@@ -96,7 +114,7 @@ genericBuild ({
         -L "$NIX_BUILD_TOP/package-build" \
         -l "$melpa2nix" \
         -f melpa2nix-build-package \
-        $ename $version $commit
+        $ename $melpaVersion $commit
 
     runHook postBuild
     '';
@@ -104,9 +122,9 @@ genericBuild ({
   installPhase = ''
     runHook preInstall
 
-    archive="$NIX_BUILD_TOP/packages/$ename-$version.el"
+    archive="$NIX_BUILD_TOP/packages/$ename-$melpaVersion.el"
     if [ ! -f "$archive" ]; then
-        archive="$NIX_BUILD_TOP/packages/$ename-$version.tar"
+        archive="$NIX_BUILD_TOP/packages/$ename-$melpaVersion.tar"
     fi
 
     emacs --batch -Q \