This is a more specific and pressing issue than T47514, which can worked avoided by using debug=true.
Even in debug mode, package files get concatenated and served from a load.php URL with a version hash that changes the URL every time the file is modified. This means that even once you've got to the source code using the convoluted method described in https://www.mediawiki.org/wiki/ResourceLoader/Package_modules#Debugging, all your breakpoints will be lost once you make a change.