How entityNamespaces and namespace registration in Wikibase (especially, with WikibaseClient enabled) changed in https://gerrit.wikimedia.org/r/#/c/337255/
This causes registration of extra entity namespaces not to work correctly.
Presumably this also affects WikibaseLexeme.
One thing we could do to improve is to define the namespaces in extension registration, instead of onSetupAfterCache hook. We also need to call WikibaseEntityNamespaces hook earlier in setup.
```
10:08:22 There was 1 failure:
10:08:22
10:08:22 1) Wikibase\MediaInfo\Tests\MediaWiki\WikibaseMediaInfoHooksTest::testOnImagePageAfterImageLinks
10:08:22 Failed asserting that '<h2><a href="/wiki/index.php/:M23" title=":M23">:M23</a></h2>' matches PCRE pattern "@<h2><a .*MediaInfo:M23.*>MediaInfo:M23</a></h2>@".
10:08:22
10:08:22 /srv/jenkins-workspace/workspace/mwext-testextension-hhvm/src/extensions/Wikidata/extensions/MediaInfo/tests/phpunit/mediawiki/WikibaseMediaInfoHooksTest.php:130
10:08:22 /srv/jenkins-workspace/workspace/mwext-testextension-hhvm/src/maintenance/doMaintenance.php:111
10:08:22
```