NamespaceInfo service to replace MWNamespace

NamespaceInfo service to replace MWNamespace

MWNamespace::clearCaches() has been removed entirely, along with the
$rebuild parameter to MWNamespace::getCanonicalNamespaces(). The rest of
MWNamespace is deprecated.

Diff best viewed with -C1 so git notices that NamespaceInfo is a copy of
MWNamespace.

Depends-On: Icb7a4a2a5d19fb1f2453b4b57a5271196b0e316d
Depends-On: Ib3c914fc99394e4876ac9fe27317a1eafa2ff69e
Change-Id: I1a03d4e146f5414ae73c7d1a5807c873323e8abc