The receiver was defined for `scrapeTargets != {} || publishedScrapeTargets
!= {}`, but the swarm-tier pipeline named it only for `scrapeTargets != {}`.
A collector with published targets and no loopback ones therefore rendered
scrape configs that belonged to no pipeline: requested, parsed, delivered
nowhere, and valid enough to deploy.
Latent, not live: the module seeds `scrapeTargets.collector` under its own
`enable`, so the loopback set is never empty in a real deploy and the
disagreeing arm is unreachable today. Nothing would have noticed if that
seeding became conditional, which is why the module-eval case pins the
pipeline rather than trusting the seed to keep masking it.
Refs #4511
fb9a2c0830