fetch relative to layouts

[Rich-Harris]

#6337. Just a failing test for now, as I realised that there's no sense proceeding until #6541 is merged.

Couple of thoughts:

• I think the only real way to make this work is to include a regex with each layout
• In Resolve relative fetch from +layout relative to the layout's route, not the leaf's route #6337 we concluded that a fetch in src/routes/foo/+layout.js should resolve relative to /foo/, not /foo. Since it would be very weird if src/routes/foo/+page.js then resolved relative to /foo, it probably makes sense for all relative fetches to resolve as though the trailing slash were there.

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[Rich-Harris]

we decided against this