Bootstrap: add a composer.json file to the project. No code relies on it (yet), and no vendor dir is checked in (yet). No autoloader is being used (yet). Taking this first step will hopefully encourage the perfectionists of our world to scrutinize each line.
To fiddle around with what it does, run composer install from the project root. If you do not have Composer installed:
https://getcomposer.org/doc/00-intro.md#locally
tl;dr for Mac/Homebrew users: brew install composer
Classes from wp-includes and wp-admin are eligible for autoloading via autoload.classmap. Through a tornado of recent commits, many unsuitable files have been transitioned into a more acceptable state for autoloading: 1 file per class, no side effects.
The file bootstrap in wp-settings.php can transition into autoload.files. This will be done with care and attention.
See #36335.