Add interwiki support to LinkTarget and TitleValue

This adds support to the LinkTarget interface and TitleValue
implementation for having an interwiki component, matching the function
names used in Title.

MediaWikiTitleCodec was updated accordingly.

The motivation behind this change is to be able to fully use LinkTarget
in the Linker rewrite instead of depending upon Title.

Change-Id: I6666b64f0e336aadc7261e7ca87ac2e498c61856