stdenv: provide meta.evaluates

This gives a way to see the result of `check-meta` without triggering any assertions.

pkgs/stdenv/generic/check-meta.nix

@@ -159,6 +159,7 @@ let
     executables = listOf str;
     outputsToInstall = listOf str;
     position = str;
+    evaluates = bool;
     repositories = attrsOf str;
     isBuildPythonPackage = platforms;
     schedulingPriority = str;

pkgs/stdenv/generic/make-derivation.nix

@@ -211,6 +211,9 @@ rec {
         # Fill `meta.position` to identify the source location of the package.
         // lib.optionalAttrs (pos != null) {
           position = pos.file + ":" + toString pos.line;
+        # Expose the result of the checks for everyone to see.
+        } // {
+          evaluates = validity.valid;
         };
 
     in