Make app packaging incremental

[tdecroyere]

Goal

Avoid re-running application dependency packaging on every build when nothing changed.

Changes

• replace always-dirty custom packaging targets with stamp-backed custom commands;
• rebuild package dependency copies only when the dependency target changes;
• chain directory copies for apps with multiple dependencies so they do not write the same package directory concurrently;
• apply the same incremental behavior to copied macOS framework dependencies;
• keep resource copies on their existing output/dependency-based incremental path.

Expected behavior

A no-op rebuild should no longer re-copy Elemental/ElementalTools into every application package. When a packaged dependency is rebuilt, only the affected package copy commands should run again.

Do not merge yet; validate on the supported generators/platforms first.