Rename MediaWiki/mediawiki-defines.txt to MediaWiki/mediawiki-extensions.txt

Preparing for adding mediawiki-skins and this needed disambiguation.

Bug: 65040
Change-Id: Ib052bc7b9535b9cc2da88f4fa0cb6862006fda21
Merge "Rename MediaWiki/mediawiki-defines.txt to MediaWiki/mediawiki-extensions.txt"