[Purity checker] Purity checker does not run as expected

[Ao-senXiong]

commands:

java -jar checker/dist/checker.jar -processor org.checkerframework.framework.util.PurityChecker Demo.java

Input:

import org.checkerframework.dataflow.qual.Pure;
public class Demo {
    Object field;;
    @Pure
    public void impureMethod() {
        field = new Object();
    }
    @Pure
    public Object pureMethod() {
        return field;
    }
}

Outputs:
Pass without issue error

Expectation:
Error on impuremethod.

Apply -AcheckPurityAnnotations will successfully issue the error. However, run PurityChecker directly should not need the flag.
Think about how to enable it by default and add some test suite.