treewide: rename version attributes

As suggested in https://github.com/NixOS/nixpkgs/pull/39416#discussion_r183845745
the versioning attributes in `lib` should be consistent to
`nixos/version` which implicates the following changes:

* `lib.trivial.version` -> `lib.trivial.release`
* `lib.trivial.suffix` -> `lib.trivial.versionSuffix`
* `lib.nixpkgsVersion` -> `lib.version`

As `lib.nixpkgsVersion` is referenced several times in `NixOS/nixpkgs`,
`NixOS/nix` and probably several user's setups. As the rename will cause
a notable impact it's better to keep `lib.nixpkgsVersion` as alias with
a warning yielded by `builtins.trace`.

lib/default.nix

@@ -58,7 +58,7 @@ let
       replaceStrings seq stringLength sub substring tail;
     inherit (trivial) id const concat or and boolToString mergeAttrs
       flip mapNullable inNixShell min max importJSON warn info
-      nixpkgsVersion mod compare splitByAndCompare
+      nixpkgsVersion version mod compare splitByAndCompare
       functionArgs setFunctionArgs isFunction;
 
     inherit (fixedPoints) fix fix' extends composeExtensions

lib/trivial.nix

@@ -58,12 +58,14 @@ rec {
 
   inherit (lib.strings) fileContents;
 
-  version = fileContents ../.version;
-  suffix = let suffixFile = ../.version-suffix; in
+  release = fileContents ../.version;
+  versionSuffix = let suffixFile = ../.version-suffix; in
     if pathExists suffixFile then fileContents suffixFile else "pre-git";
 
   # Return the Nixpkgs version number.
-  nixpkgsVersion = version + suffix;
+  version = release + versionSuffix;
+
+  nixpkgsVersion = builtins.trace "`lib.nixpkgsVersion` is deprecated, use `lib.version` instead!" version;
 
   # Whether we're being called by nix-shell.
   inNixShell = builtins.getEnv "IN_NIX_SHELL" != "";