FTS index table not have to be qualified by data source alias in query

For function match(), the first argument refers to the FTS index table. Syntactically, it appears as a property path. The leading part may be the alias of a data source. It is required if there multiple data sources. In Lithium, it is necessary for join, but for FTS match, it is unnecessary because we only need to know what database table this index is associated with, and, in Lithium, there is only one table.

Therefore we can have it to be optional: if not prepended by alias, we use the default alias.