correctly determine whether a route uses server data

[Rich-Harris]

fixes #6269
fixes #6258
fixes #6276
fixes #6257

Prior to this change, the logic for determining whether a given route uses server data (and therefore needs to hit __data.json on invalidation/navigation) was faulty if the only server data comes from a sibling +layout.server.js.

The test is a bit of a sleight-of-hand because there's already a parent +layout.server.js, so it was already being picked up. But I've tested this fix in a repro and it works correctly.

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