As per the documentation (see "(elisp)Startup Summary"), only variable
customization that affect package initialization should go into the early init
file. Defining package archives is explicitly mentioned as something that still
may go into the main init file. So we move it there and adjust comments
accordingly.
This is some relic from former configurations, and might have been wrong quite
some time now. However, in Emacs 27+, package initialization is done before
loading user-init-file, and thus we don't have to do it by hand. Before that,
package initialization was done after reading user-init-file, but before calling
after-init-hook, and since we needed to load some packages in the init file, we
had to initialize package.el ourselves.
Daniel -