MessageCache: Restore 'loadedLanguages' tracking for load()

MessageCache: Restore 'loadedLanguages' tracking for load()

This was removed in 97e86d934b3 in 2018 in favour of using
`$this->cache->has($code)`. This is a problem because there
are cases where only a narrow subset of that structure is
populated (by MessageCache->replace) without things like
$this->overridable (or anything else that MessageCache->load does)
having ocurred yet.

The assumption that keys are only added to $this->cache by
MessageCache->load (or after that method has been called) was
actually true at some point. But, this changed in 2017 when
commit c962b480568e optimised MessageCache->replace to not call
MessageCache->load.

Bug: T208897
Change-Id: Ie8bb4a4793675e5f1454e65c427f3100035c8b4d