Title: Simplify use of MWNamespace::getCanonicalName()

Title: Simplify use of MWNamespace::getCanonicalName()

The exists() check performs most of what getCanonicalName() will
do a second time. Make the code more simple, DRY, and performant,
by aiming for EAFP instead of LBYL.

Also fixes a potential bug where it was using current namespace
from $this for exists(), but subject namespace (stored as $namespace)
for getCanonicalName().

Change-Id: I11539bde00bf3df3270869f40fbd62083588b50c