So I found my own solution.
package classes must be in a directory with a name corresponding to the package declaration. Moving the files into a foobar directory eliminated the runtime exception.
Note, however that
java must be invoked from the root level. Thus, if one runs java Moo from within the foobar directory, the runtime exception occurs. If one steps back to the parent directory and runs java foobar/Moo it works properly.
Mark