Prefer `checkstyle.xml` from project folder

If the project (root) folder contains a `checkstyle.xml` file, automatically prefer that configuration over the global check configurations. "Automatically" so that the user does not need to setup anything despite the installation of the Eclipse Checkstyle plugin.

If the Eclipse Checkstyle plugin is installed in Eclipse and the project root folder contains a `checkstyle.xml` file, then the Eclipse Checkstyle plugin should automatically load the checkstyle rules from the file to check the code in Eclipse for the project that has the file.