#47256 Use composer to install and update external PHP libraries

[johnbillion]

This introduces a custom Composer installer plugin in order to facilitate installing some third party libraries via Composer into the wp-includes directory. This is needed because none of the existing Composer installer libraries support features such as ignoring files or installing just a subdirectory of the package, both of which are needed for WordPress.

Once this is implemented, updating one of these dependencies will be a case of:

1. Change the required version number in composer.json
2. Run composer update
3. Check and commit the changed files

Ticket

Trac ticket: https://core.trac.wordpress.org/ticket/47256

Done

• PHPMailer
• sodium_compat

In progress

• Requests
  • Need to handle the fact that Requests/library/Requests.php is not actually the file from Requests
  • Need to address the fact that security hardening was applied in core and never backported to Requests
• SimplePie
  • Need to investigate why there is a difference in src/IRI.php
• ID3
  • Need to handle its license and readme

N/A

• Text_Diff
  • No matching version on Packagist
• Pomo
  • Pomo in core is heavily customised AFAICT
• random_compat
  • No longer needed