lib.trivial.warnIf: init

It's a common pattern in Nixpkgs to want to emit a warning in certain
cases, but not actually change behaviours.

This is often expressed as either
	if cond then lib.warn "Don't do that thing" x else x
Or
	(if cond then lib.warn "Don't do that thing" else lib.id) x

Neither of which really expresses the intent here, because it looks
like 'x' is being chosen conditionally.

To make this clearer, I introduce a "warnIf" function, which makes it
clear that the only thing being affected by the condition is whether
the warning is generated, not the value being returned.

lib/default.nix

@@ -66,8 +66,9 @@ let
       stringLength sub substring tail trace;
     inherit (self.trivial) id const pipe concat or and bitAnd bitOr bitXor
       bitNot boolToString mergeAttrs flip mapNullable inNixShell isFloat min max
-      importJSON importTOML warn info showWarnings nixpkgsVersion version mod compare
-      splitByAndCompare functionArgs setFunctionArgs isFunction toHexString toBaseDigits;
+      importJSON importTOML warn warnIf info showWarnings nixpkgsVersion version
+      mod compare splitByAndCompare functionArgs setFunctionArgs isFunction
+      toHexString toBaseDigits;
     inherit (self.fixedPoints) fix fix' converge extends composeExtensions
       composeManyExtensions makeExtensible makeExtensibleWithCustomName;
     inherit (self.attrsets) attrByPath hasAttrByPath setAttrByPath