flake/version overlay: review fixes

* Improves the comments of `lib/flake-version-info.nix` and drops the
  `__`-prefix from the filename.
* `lib'` -> `lib0` in `nixpkgs/lib`.
* Drop the declaration of `trivial.version` in the overlay because this
  declaration already uses the final expressions of `versionSuffix` and
  `release` now.
* No need to fall back to `self.lastModified` anymore, this was a
  workaround for pre2.4 Nix.

Co-authored-by: Robert Hensing <robert@roberthensing.nl>
Co-authored-by: Silvan Mosberger <contact@infinisil.com>

flake.nix

@@ -9,7 +9,7 @@
         nixpkgs = self;
       };
 
-      libVersionInfoOverlay = import ./lib/__flake-version-info.nix self;
+      libVersionInfoOverlay = import ./lib/flake-version-info.nix self;
       lib = (import ./lib).extend libVersionInfoOverlay;
 
       forAllSystems = lib.genAttrs lib.systems.flakeExposed;

lib/__flake-version-info.nix

@@ -1,17 +0,0 @@
-# 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-version-info.nix

@@ -0,0 +1,20 @@
+# This function produces a 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!
+# Instead, consider using a public interface, such as this flake here
+# in this directory, `lib/`, or use the nixpkgs flake, which applies
+# this logic for you in its `lib` output attribute.
+
+self: # from the flake
+
+finalLib: prevLib: # lib overlay
+
+{
+  trivial = prevLib.trivial // {
+    versionSuffix =
+      ".${finalLib.substring 0 8 (self.lastModifiedDate or "19700101")}.${self.shortRev or "dirty"}";
+    revisionWithDefault = default: self.rev or default;
+  };
+}

lib/flake.nix

@@ -3,8 +3,8 @@
 
   outputs = { self }:
     let
-      lib' = import ./.;
+      lib0 = import ./.;
     in {
-      lib = lib'.extend (import ./__flake-version-info.nix self);
+      lib = lib0.extend (import ./flake-version-info.nix self);
     };
 }