[fix] resolve fetch requests from the target page

[mrkishi]

When using load's fetch, it's natural to assume URLs will be resolved relatively to the navigation's target page. Instead, they take the current page into account. This means that, in practice, we must always use absolute or root-relative URLs.

This PR resolves URLs relatively to the target page before issuing the native fetch. It's a simple fix, with just a couple of wrinkles we must pay attention to:

• The Request object is mostly read-only, so we need to consume its body and manually copy properties into the new request;
• Our ssr-included payloads wouldn't match the full URLs, so we skip normalization during hydration.

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Latest commit: f5bb10a

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@sveltejs/kit	Patch

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