I also tried to find a way to identify all packages in a way similar to
list-packages, but it seems not all libraries are considered as packages so they are not listed among other packages.
There are a few ways of identify what is built-in into Emacs:
- Official Manuals which cover a lot of things, including manuals for various packages including RCIRC: GNU Emacs Manuals Online
lisp/ directory of your Emacs installation or CVS tree online (e.g., master of emacs-27), find .el files and look at the header. You can automate this stuff with grep like this (you might need to tweak this regexp):
grep -r -E ';;;.*\.el\s+---\s+.*' <path-to-emacs-installation>/lisp
The output gives about 1400 lines (emacs-26.1.50), some of which are just libraries or minor modes. For example, here's the output for rcirc:
net/rcirc.el:;;; rcirc.el --- default, simple IRC client