Interpret /hst:hst in auto-export config to match any HST root node

For developer convenience, we should treat auto-export config that defines any rule referencing the literal "/hst:hst" root node as if it were applicable to any HST root node. This will allow for the default auto-export behavior to work as expected for any site project, regardless of the actual HST root node, without repeating the full configuration used for /hst:hst.

Note that it would be acceptable (and likely required) for site devs to provide some form of config to register the HST root node with auto-export.