make it possible to control partial ordering of layers

When transitioning to data in environments and when dealing with different versions of modules (with various degree of quality of their data). It is of importance to be able to define that entries in a global or environment hiera.yaml has lower precedence than those normally having lower precedence.

While these can be achieved with PUP-7332 as a kind of filter it makes it difficult to do the same with an existing data function. The typical example here would be to have a common layer in the global hiera.yaml that kicks in after the environment, or an entry in an environment that kicks in after modules. It is meaningless to have this in a module's hiera.yaml.

An easy way to configure this is to add a kind of function called layer that accepts environment or modules. If this is present in a hierarchy the lower level is visited at that point in the higher layer's hierarchy. If not present, the lower precedence layer is automatically visited after the higher layer's precedence (as before this ticket).

The entry layer: environment is only allowed in a global hiera.yaml, and the entry layer: modules is only allowed in an environment's hiera.yaml. The module layer accepts none of those.