Follows-up Ib4caa80bb700, which introduced MW_INSTALL_PATH as
alternative to $IP, but also introduced $wgBaseDirectory.
Its not clear to me whether we actually want/need $wgBaseDirectory
long-term as it introduces significant complexity for a distinction
that remains theoretical, not enforced, and lacking a use case.
But, at least here we don't need it as these all explicitly reference
files that are in the source tree. Mocking the path differently would
fail the tests.
There are some individual methods (OutputPage, used via SkinModule)
that do not make assumptions on file paths and work with any file
paths at runtime. Those indeed need $wgBaseDirectory for DI and
testability.
Change-Id: I56c28d36e2d7bbf7ec5a8f2f5ae99b291e7ee4e8
e446a71f86