$wgAPIModules should extend ApiBase, not ApiQueryBase

I have no idea why this was extending ApiQueryBase since it doesn't use
any methods defined in that class. But at least that makes it easy to
fix.

Bug: 65537
Change-Id: Id11582f2b81db26bc9f87ef7380db67e308bb04e
Rename ApiQuerySiteMatrix class to ApiSiteMatrix

"ApiQueryFoo" is normally used for submodules of action=query. May as
well rename this to follow that standard naming convention.

Change-Id: I988e0b73edd2ffff96a0c8c4c0b437291ca6ca21