Insight into environment/configuration variables

[mosteo]

The kind of problems being addressed in https://github.com/alire-project/alire-index/pull/1082 made me think about this.

It would be nice to have a way of seeing all configuration and environment variables and what/where are being being set. Similar to how with alr printenv you can see all paths, but tailored to the parameters that affect the build. More rationale:

We don't normally want libraries to set their own variables (unless it is absolutely univocal due to environment), so there may be unset gpr variables that require setting, or whose defaults may be not always prefereable. Currently, you cannot get this information for all releases in the solution in a simple way, as it is scattered through manifests.

Something like alr build --scenario that gave this kind of output:

gnoga=2.1.2
simple_components=4.68.0
   target_os (gpr var) := Linux (set by mycrate, gnoga)
mycrate=0.1.0-dev
   flavor (config var) in (Wayland, X11) (unset)

It should show what is unset with its possible values, what is set in the environment, and what is set by a crate or more