When upgrading from MW 1.24 to 1.26.2, I'm running into this error:
MediaWiki 1.26 internal error
Installing some external dependencies (e.g. via composer) is required.
External dependencies
MediaWiki now also has some external dependencies that need to be installed via composer or from a separate git repo. Please see mediawiki.org for help on installing the required components.
I've installed Composer and run the installation and upgrade commands, but nothing seems to fix this. How do I install these external dependencies? The documentation on this is difficult to understand.