standalone application build script fails when the source directory contains more than 1 class

This happens in certain cases, depending on the name of the classes.

For instance, in s4-piper-app, if we add a class named HelloApp2 (that does nothing), the build fails, notably saying:
16:57:52.033 [QUIET] [system.out] Found app class name: s4app.HelloApp
16:57:52.039 [QUIET] [system.out] Couldn't find App class in source files...aborting.