Finalize `pkgEnt` and default `pkgSet` pipeline

[aameen-tulip]

The branch mk-src-ent has made a big push on this, and aims to create a formal spec for pkgEnt. This will help land on a common standard for package definitions to be shared in overlays across projects.

While pkgSet pipelines necessarily need to be more flexible, a set of interfaces implemented by different backends should help scaling small clusters centered around package locks and workspaces to be compatible with larger hierarchical sets with multiple versions like we have in flocoPackages.

addFlocoPackage[s] ( overlay generator ), getFlocoPackage ( lookup by exact ident+version ), resolveFlocoPackages ( filters by semver ), and similar interfaces can abstract a packument VInfo hierarchy vs. a keyed set. Definitions for these already exist for the keyed metaSet backend.