Javac option to change source path to find

Hello! I wrote about a problem I had with javac in a discussion thread, and just now I answered it myself.

The problem was that javac wasn't able to find the source files, so all imports within my own package were giving errors.
The fix was changing
`let l:src_dir = ale#path#FindNearestDirectory(a:buffer, 'src/main/java')` to `let l:src_dir = ale#path#FindNearestDirectory(a:buffer, 'src')`
I think it would be really nice if I could specify this option in the configuration file, instead of having a modified version of ALE

I saw a javac ALE option called "sourcepath" but I couldn't get it to work, and from what I understood is to specify a direct sourcepath - this new option would be to configure the auto search for a `src/`

Discussion:
https://github.com/dense-analysis/ale/discussions/3677