Decouple LayerEnv from implicit CNB layer env rules

[Malax]

LayerEnv is a representation of environment modifications as per the CNB spec. There are also some implicit changes to the environment in the CNB spec to automatically add $layer/bin, $layer/lib, $layer/include and $layer/pkgconfig for each layer.

Currently, LayerEnv::read_from_layer_dir does not only read the explicit values from disk, but also adds the implicit ones from the spec. One can make an argument that this behaviour is correct if we define the implicit values as always present and overwritten by explicit rules, but there are some issues with the current implementation in practice.

The simplest case would be reading LayerEnv from disk and then persisting it to disk without changes. This currently leads to the situation that all the implicit values are written to disk explicitly, changing the state. This is unexpected behaviour and may lead to subtle bugs.

This issues proposes a solution to the problem by removing all handling of the implicit values from LayerEnv. This would make LayerEnv a strict representation of what is present on disk without involving "magic".

However, buildpacks will often need a LayerEnv that does include the explicit entries for subsequent logic, most commonly invoking binaries stored in $layer/bin by searching PATH. We still should make it easy to modify an existing Env (not LayerEnv!) as if the layer was processed by the CNB lifecycle.

For this use-case, we can provide a function on LayerRef with a signature like this:

fn apply_layer_env(&self, &mut Env)

This function would use the LayerEnv data from disk but also the implicit rules defined in the CNB spec. To clarify, read_layer_env would still only return the data from disk, cleanly separating both concerns.

In addition, we should rename LayerEnv to EnvDelta (or similar), clarifying the meaning of the represented data. In addition, LayerRef::write_env would be renamed to LayerRef::write_env_delta. The same applies to LayerRef::read_env_delta.