DbView: recipes and layers
- get layer (and possible overrides coming from other layer) for a recipe
- get all recipes (and overrides) belonging to a layer
- get all recipes (and overrides) grouped by layer
In each of the views above, layer name should be accompanied by the git repo url and revision (or other upstream provenance data) where that layer comes from.
Rationale: recipes are organized in layers, which come from git repos (one git repo may contain one or more layers). If a recipe introduces a technical or legal issue, one should be able to dectect which upstream git repo to blame (it could be the "main" layer where the recipe comes from, or another layer that overrides something in that recipe)