The Java source launcher executing a single source file that with a shebang line should ignore any module-info.java file present in the program's root directory. As per JEP 458:

    Step 1:
    If the file begins with a "shebang" line, that is, a line that starts with #!,
    then the source path passed to the compiler is empty so that no other
    source files will be compiled. Proceed to step 4.

An error message can read like this:

    module-info.java:4: error: file should be on source path, or on patch path for module
    module helper {
    ^
    1 error
    error: compilation failed

In such cases, no error should be raised and the program be run on the classpath.