chore(eslint-plugin): linter chokes on aws-cdk-lib/core import (#26022)

#25802 changed imports in alpha modules from `aws-cdk-lib` to `aws-cdk-lib/core`.

Not sure how that got past the linter in the first place, but my linter is choking on this because `aws-cdk-lib/core` does not have a `package.json`.

```bash
Error: ENOENT: no such file or directory, open '<path>/aws-cdk/packages/aws-cdk-lib/core/package.json'
```

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

tools/@aws-cdk/eslint-plugin/lib/rules/invalid-cfn-imports.ts

@@ -143,6 +143,10 @@ function getCdkRootDir(filename: string): string | undefined {
 }
 
 function isAlphaPackage(packageDir: string): boolean {
+  if (packageDir.endsWith('aws-cdk-lib/core')) {
+    return false; // special case for core because it does not have a package.json  
+  }
+
   const pkg = JSON.parse(fs.readFileSync(path.join(packageDir, 'package.json'), { encoding: 'utf-8' }));
 
   const separateModule = pkg['separate-module'];
@@ -182,4 +186,4 @@ function checkLeftAndRightForCfn(node: any): { name: string, location: string }
   const left = checkLeftAndRightForCfn(node.left);
 
   return right ?? left ?? undefined;
-}
+}