Fix cross-export warning calculation:

Author: srawlins

lib/src/model/model_element.dart

@@ -541,9 +541,10 @@ abstract class ModelElement extends Canonicalization
 
     // Avoid claiming canonicalization for elements outside of this element's
     // defining package.
-    // TODO(jcollins-g): Make the else block unconditional.
+    // TODO(srawlins): Make the else block unconditional, so that we always
+    // exclude cross-package exports.
     if (candidateLibraries.isNotEmpty &&
-        !candidateLibraries.any((l) => l.package == definingLibrary.package)) {
+        candidateLibraries.any((l) => l.package != definingLibrary.package)) {
       warn(PackageWarning.reexportedPrivateApiAcrossPackages,
           message: definingLibrary.package.fullyQualifiedName,
           referredFrom: candidateLibraries);