Add `collectFileChanges()` next to the other repository helpers, plus a
thin `fileChangesToArtifactResults()` in `lib/modules/manager/util.ts`,
and migrate the managers which re-implemented the same status loops:
apm, cocoapods, composer, copier, gomod, helmv3, jsonnet-bundler,
kustomize and vendir.
Each site keeps its own status bucket selection and path filter, so the
emitted artifacts are unchanged. Two dead pieces of logic are dropped:
vendir's `vendorDir || isFileInVendorDir` condition, which was always
true, and kustomize's optional chaining on a non-nullable status, which
only ever mattered for an unset test mock (now set explicitly).
Sebastian Poxhofer