Does installing via Composer, `extension.json`, and `$wgExtensionDirectory` work?

[mwjames]

For some context, see https://phabricator.wikimedia.org/T139853.

[s7eph4n]

Good question.
Don't currently have the time to test this. In fact I will leave for a month day after tomorrow, so it would be great if somebody else could confirm the issue.