fix: make route resolution imports root-relative if path.relative option is false

[dummdidumm]

It is necessary to adhere to this setting because rewrites that SvelteKit cannot see may otherwise lead to incorrect relative paths.

Example:

• route resolution request to _app/route.js
• rewrite to _app/route/foo.js
• SvelteKit only sees the later, returning a relative import('../immutable/...) which is wrong, because the actual relative path would be import('./immutable/...)

Tested manually that it works correctly; automated test feels unnecessary

---

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