Fix project detection in root Jest config

Prior to this, empty directories in `packages` would have hindered Jest
from transforming some files, resulting in "unexpected token" errors

Author: bjoluc

package.json

@@ -41,7 +41,7 @@
   },
   "jest": {
     "projects": [
-      "<rootDir>/packages/*"
+      "<rootDir>/packages/*/jest.config.cjs"
     ]
   }
 }