Support YAML anchor <<: syntax

Currently most of our components depend on package `config` as that's where their configuration structs are defined. That's noisy for package reuse. In some places we work around it by specifying structs in the packages and mirroring them. Package `config` should be similar to `main` in that it bundles together all packages and parses the YAML configuration into the package's own confiuguration structs. We should also get rid of the YAML-unmarshal-embedded validation and instead provide decicate `ValidateX` methods.