Some, but not all extensions use their own PHP namespacing.
Basic process. See https://gerrit.wikimedia.org/r/#/c/398320/ for an example of some of these steps
* Swap `extension.json` to use `AutoloadNamespaces` instead of `AutoloadClasses`
* Under `"requires"`, set the `"MediaWiki"` version requirement to `>= 1.31.0`
* Move any PHP files deemed necessary
* [PHPCodeSniffer](https://www.mediawiki.org/wiki/Continuous_integration/PHP_CodeSniffer) helps us stick to same coding standards across MediaWiki and it's extensions. This is still work in progress, and some sniffs are still excluded in the file `.phpcs.xml` (and those exclusions should get removed, in the long run). Disable `MediaWiki.Files.ClassMatchesFilename.NotMatch`, `MediaWiki.Files.ClassMatchesFilename.WrongCase` and `MediaWiki.Files.OneClassPerFile.MultipleFound` in the file `.phpcs.xml` if these entries exist as exclusions.
* Fix any resultant test failures from any of the changes above.