Produce several component-scoped packages from one build in reusable

[alexadereyko]

Depends on:

• Add staging-cleanup and exercise it on the test packages #10

Brief

Let a reusable job declare its outputs as lists, so one build produces several component-scoped packages: stagings: [{name, variables}] and, likewise, installers. Each named package cpacks into its own <staging|package-dir>/<name>/ subdir of the single staging-<job> / package-<job> artifact.

Description

cpack-presets moves from a single staging + single installer per job to two lists:

• stagings — TGZ tarballs, one per {name, variables, package-preset}.
• installers — native packages, one per {name, package, variables, package-preset}.

Named packages merge across matching entries (variables update, package / package-preset last-wins), so a ["*"] entry sets common variables and a platform entry adds its installer. Per package a <job>-staging-<name> / <job>-package-<name> packagePreset cpacks into <staging|package-dir>/<name>/; the whole dir uploads as the one staging-<job> / package-<job> artifact. staging-push already globs the nested staging-*/<name> layout, so it needs no change.

Test

test-staging folds into two jobs:

• publish — one umbrella hello build emits both the core and module stagings and installers and publishes each to its own ghcr index. Producing several component packages from one build is the check.
• consume — pulls hello-core back from ghcr and builds hello-module against it, without publishing.