Feature request: `do-like-javac` should support projects with a subdirectory structure

Summary

Currently, running wpi.sh on a project with a subdirectory structure (i.e., having one or more subdirectories which are themselves compilable units) appears to be failing with an error related to do-like-javac.

For example, the project below

https://github.com/Javacord/Javacord f8eedf6c6a9c61d2d74b28202a370c9cccd175cc

Compiles without warning using the compile-project script from manage-git-branches.

However, when wpi.sh is executed from the root of the project:

% wpi.sh -- --checker optional

The script immediately returns with the following output:

Starting wpi.sh.
Finished configuring wpi.sh.
no build file found for ; not calling DLJC
First run configure_and_exec_dljc with JAVA_HOME=/homes/gws/jmsy/opt/jdk/jdk-11.0.19: WPI_RESULTS_AVAILABLE=no build file found for ; not calling DLJC
wpi.sh: dljc could not run the build successfully: no build file found for ; not calling DLJC
Check the log files in /scratch/jmsy/Javacord/dljc-out/ for diagnostics.
Exiting wpi.sh successfully; pwd=/scratch/jmsy/Javacord

And the dljc-out/ folder mentioned in the message is not produced.
The script does say

First run configure_and_exec_dljc with JAVA_HOME=/homes/gws/jmsy/opt/jdk/jdk-11.0.19: WPI_RESULTS_AVAILABLE=no build file

But I am unsure if that is related or a red herring.