Add checkDependencies.php

Checks to see whether the specified MediaWiki extensions/skins have all of
their dependencies. Each thing will get a status of:

* 'loaded' (it's already activated in LocalSettings.php, and therefore all
  the dependencies are present)
* 'present' (it exists in the filesystem, but not yet activated)
* 'missing' (does not exist in the filesystem)
* 'incompatible-*' (some version requirement is incompatible)

JSON output is available with --json for use in scripts.

Development dependencies will be checked if --dev is provided.

This is roughly based on the logic already existing in the Installer. It may
be worth deduplicating that code in the future.

Bug: T193824
Change-Id: Iae9178225c10b6374484274d8848bbea6621c8c2