flake: also provide proper version info for lib's flake

This effectively means that

    nixpkgs$ nix eval ./lib#lib.trivial.version
    "23.11.20231020.ee0d6b5"

now gives meaningful results as well.

See https://github.com/NixOS/nixpkgs/pull/257100#discussion_r1352075369
for the discussion around this.

flake.nix

@@ -9,17 +9,9 @@
         nixpkgs = self;
       };
 
+      libVersionInfoOverlay = import ./lib/__flake-version-info.nix self;
       lib = (import ./lib).extend libVersionInfoOverlay;
 
-      libVersionInfoOverlay = finalLib: prevLib: {
-        trivial = prevLib.trivial // {
-          versionSuffix =
-            ".${finalLib.substring 0 8 (self.lastModifiedDate or self.lastModified or "19700101")}.${self.shortRev or "dirty"}";
-          version = finalLib.trivial.release + finalLib.trivial.versionSuffix;
-          revisionWithDefault = default: self.rev or default;
-        };
-      };
-
       forAllSystems = lib.genAttrs lib.systems.flakeExposed;
     in
     {

lib/__flake-version-info.nix

@@ -0,0 +1,17 @@
+# lib overlay to be used by the nixpkgs & nixpkgs/lib flakes
+# to provide meaningful values for `lib.trivial.version` et al..
+#
+# Internal and subject to change, don't use this anywhere else!
+
+self: # from the flake
+
+finalLib: prevLib: # lib overlay
+
+{
+  trivial = prevLib.trivial // {
+    versionSuffix =
+      ".${finalLib.substring 0 8 (self.lastModifiedDate or self.lastModified or "19700101")}.${self.shortRev or "dirty"}";
+    version = finalLib.trivial.release + finalLib.trivial.versionSuffix;
+    revisionWithDefault = default: self.rev or default;
+  };
+}

lib/flake.nix

@@ -1,5 +1,10 @@
 {
   description = "Library of low-level helper functions for nix expressions.";
 
-  outputs = { self }: { lib = import ./.; };
+  outputs = { self }:
+    let
+      lib' = import ./.;
+    in {
+      lib = lib'.extend (import ./__flake-version-info.nix self);
+    };
 }