Throw on block slugs that collide with collection or global slugs

[janbuchar]

• adjustRefTargets resolves #/definitions/<slug> by trying collections, then globals, then blocks. Payload puts all three in one namespace, so a config-level block sharing a slug with a collection silently resolves to the collection schema.
• Repro: collection pages with labels: { singular: 'Sheet' } plus a config-level block pages referenced through blockReferences — the blocks array is documented as oneOf: [$ref Sheet], i.e. as holding collection documents. Predates feat: Add blockReferences support #60; that PR only adds an orphaned Pages schema next to the same wrong ref.
• Nothing in the ref distinguishes a block from a collection, so the fix is to refuse the config rather than guess — same treatment defineSchemas gives colliding component names.