I believe the core packages are all in
atom/app-x.xx.x/resources/app.asar, which can be easily extracted if you install the
asar tool. However, that’s not something you need or want to do, since the source code is all available online, the GitHub repos have ongoing development work and the current stable versions, and if you want to override a core package, Atom makes it very easy to do so. All you need to do to override a core package is to download the package code to your computer (this is easiest and most powerful if you use
git) and then, in a command line window pointing at the directory where you have stored the package, use
apm link. APM will create a symbolic link inside
.atom/packages/ that points at the folder you’re currently in, and if it’s a valid package with the same name as a core package, Atom will load your package instead of the core package.
This is not something that novice users should engage in, but for people with enough comfort with Atom to “void the warranty” and identify when their changes are the ones causing problems, it’s a very powerful tool for customizing the editor. I’ve used it to contribute to a couple of core Atom packages when there was a change that I felt comfortable making.