Define MW_INSTALL_PATH constant and BaseDirectory config.
Application logic should use the BaseDirectory config variable.
Framework code should use MW_INSTALL_PATH to locate files should.
NOTE: Update https://www.mediawiki.org/wiki/Manual:$IP
Bug: T300301
Depends-On: I7142af16d692f26e90673b058029f572c1ea3991
Change-Id: Ib4caa80bb7007c4c7960a2fd370cf5da7d9ba344