installer: Fix display of UPGRADE by disabling InterwikiLookup

installer: Fix display of UPGRADE by disabling InterwikiLookup

Since 129067c907ea65f62, parsing section titles has looked up interwiki
prefixes with InterwikiLookup. In the web upgrader, this triggers
database access, and since that service is disabled, it throws
exceptions, causing parsing to fail.

Work around that by using a dummy InterwikiLookup service that knows
about no interwiki prefixes. Maybe one could be written to just read
from the stock `interwiki.list`, but that's a project for another time.

Bug: T196607
Change-Id: I13485a9af79297b552a1128240cb8597c2ef83d8