Reset Title cache when importing titles.

WikiImporter now uses NaiveImportTitleFactory, which in turn uses Title::makeTitleSafe,
bypassing the internal title cache. To avoid (potentially cached) Title objects obtained
via Title::newFromText getting out of sync, WikiImporter now clears the title
cache in addition to clearing the LinkCache.

NOTE: a test for this is provided by I2be12fa7d439b.

Bug: T89307
Change-Id: Ib50c48d4797fc21c62090c0be69e87f7e7d07428