Use outside settings object to fetch workers

As discussed in #1122, #1111, and in
w3c/ServiceWorker#889 (comment),
a number of problems are caused by the current setup of using the
settings object of the worker itself as the fetch client. Instead, we use
the incumbent settings object to do the fetching. (Incumbent, instead
of e.g. current, because almost everything else in the (Shared)Worker
constructors uses the incumbent settings object.)

Notably, for fetching module script trees, this necessitates separating
the fetch client settings object from the one used for the module map.

This fixes #1111 since now module workers are fetched with the correct
client, and thus automatically get the correct referrer. Dependencies,
however, require further work which will happen as part of #1150.