lib/debug: deprecate addErrorContextToAttrs

The function isn’t used anywhere and `addErrorContext` is an undocumented
builtin.
The builtin is explicitely qualified at its two uses in the module system.

lib/debug.nix

@@ -11,10 +11,6 @@ in
 
 rec {
 
-  inherit (builtins) addErrorContext;
-
-  addErrorContextToAttrs = lib.mapAttrs (a: v: lib.addErrorContext "while evaluating ${a}" v);
-
   traceIf = p: msg: x: if p then trace msg x else x;
 
   traceValFn = f: x: trace (f x) x;
@@ -98,6 +94,12 @@ rec {
           + "and will be removed in the next release." )
     (if c x then true else traceSeq (showVal x) false);
 
+
+  addErrorContextToAttrs = attrs:
+    trace ( "Warning: `addErrorContextToAttrs` is deprecated "
+          + "and will be removed in the next release." )
+    (lib.mapAttrs (a: v: lib.addErrorContext "while evaluating ${a}" v) attrs);
+
   /* Evaluate a set of tests.  A test is an attribute set {expr,
      expected}, denoting an expression and its expected result.  The
      result is a list of failed tests, each represented as {name,