doc: move package.import content higher

This is currently at the end of the doc, it likely should be found
right after the documentation about `package.exports`. Refactored the
docs while duplicating content as little as possible.

Co-authored-by: Guy Bedford <guybedford@gmail.com>
Co-authored-by: Antoine du Hamel <duhamelantoine1995@gmail.com>
Co-authored-by: Rich Trott <rtrott@gmail.com>
Signed-off-by: Myles Borins <mylesborins@github.com>

PR-URL: https://github.com/nodejs/node/pull/35535


Reviewed-By: Guy Bedford <guybedford@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ujjwal Sharma <ryzokuken@disroot.org>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>