Proposal: Move Solus to serpent-based infrastructure (better, faster build- and rebuild-story via multiple builder support)

[ermo]

@ikeycode and @ermo are generously offering to update the existing, working Serpent infrastructure to accommodate Solus.

The idea is to build tooling that will add serpent-format manifests next to each existing package.yml recipe + pspec xml file in the Solus monorepo.

This tooling will be able to be invoked both from inside solbuild and on the entire recipe tree. This is necessary because before Summit (the Serpent build controller) can begin planning builds, it needs to have an initial "seed state" for the Solus packages/ git repository, for it to be able to observe the difference in manifests between what exists in the git recipe tree vs. what exists in the binary package repo.

This would also serve as a way to begin the journey towards proper convergence and Solus 5.

As an additional goal, the increased information captured in the serpent-format manifests is also likely to enable better tree-wide analysis for various consistency issues, thus increasing reliability and the level of insight into the actual state of the Solus recipe repo tree.

We also envision that autobuild will be adapted to use these new manifests for even better dependency resolver functionality for local rebuilds, when preparing PRs for submission to the monorepo.