lib: Factor in tiny bit of `meta.platform` checking

I need it in stdenv and release-lib, so that seems motivation enough.

lib/meta.nix

···
   */
   hiPrioSet = set: mapDerivationAttrset hiPrio set;
 
+
+  /* Check to see if a platform is matched by the given `meta.platforms`
+     element.
+
+     A `meta.platform` pattern is either
+
+       1. (legacy) a system string.
+
+       2. (modern) a pattern for the platform `parsed` field.
+
+     We can inject these into a patten for the whole of a structured platform,
+     and then match that.
+  */
+  platformMatch = platform: elem: let
+      pattern =
+        if builtins.isString elem
+        then { system = elem; }
+        else { parsed = elem; };
+    in lib.matchAttrs pattern platform;
 }

pkgs/stdenv/generic/check-meta.nix

···
     else "key '${k}' is unrecognized; expected one of: \n\t      [${lib.concatMapStringsSep ", " (x: "'${x}'") (lib.attrNames metaTypes)}]";
   checkMeta = meta: if shouldCheckMeta then lib.remove null (lib.mapAttrsToList checkMetaAttr meta) else [];
 
-  checkPlatform = attrs: let
-      raw = attrs.meta.platforms;
-      uniform = map (x: if builtins.isString x then { system = x; } else { parsed = x; }) raw;
-    in lib.any (pat: lib.matchAttrs pat hostPlatform) uniform;
+  checkPlatform = attrs:
+    lib.any (lib.meta.platformMatch hostPlatform) attrs.meta.platforms;
 
   # Check if a derivation is valid, that is whether it passes checks for
   # e.g brokenness or license.

pkgs/top-level/release-lib.nix

···
   packagePlatforms = mapAttrs (name: value:
     let res = builtins.tryEval (
       if isDerivation value then
-        # TODO(@Ericson2314) deduplicate with `checkPlatform` in
-        # `pkgs/stdenv/generic/check-meta.nix`.
         value.meta.hydraPlatforms or (let
-            raw = value.meta.platforms or [ "x86_64-linux" ];
-            toPattern = x: if builtins.isString x
-                           then { system = x; }
-                           else { parsed = x; };
-            uniform = map toPattern raw;
-            pred = hostPlatform:
-              lib.any (pat: lib.matchAttrs pat hostPlatform) uniform;
-            pred' = system: pred (lib.systems.elaborate { inherit system; });
-         in lib.filter pred' supportedSystems)
+            linuxDefaulted = value.meta.platforms or [ "x86_64-linux" ];
+            pred = system: lib.any
+              (lib.meta.platformMatch (lib.systems.elaborate { inherit system; }))
+              linuxDefaulted;
+          in lib.filter pred supportedSystems)
       else if value.recurseForDerivations or false || value.recurseForRelease or false then
         packagePlatforms value
       else