config: finalize and version format for v0.1

apkg relies on reading its distro/config/apkg.toml config in order to work correctly so its compatibility must be ensured through time. This is especially important in --upstream mode which uses distro from archives - compatibility must be maintained in order for apkg to build packages from older archives.

• rethink and refactor apkg.toml before %v0.1
• implement config versioning to allow config changes without breaking older configs