Update autoload classmap for Symfony runtime and etc.

Problem/Motivation

Drupal includes some files in the autoload classmap section of core/composer.json that we know are loaded on every request - this avoids having to locate the files and cache them in APCu.

We haven't updated this list for a while, and it definitely doesn't include Symfony runtime and related files recently added which are some of the first classes to be loaded during any bootstrap.

Steps to reproduce

Proposed resolution

On a page cache hit, see which files go via the classloader findFile method and add them to the classmap.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet