Assign priorities to configuration scopes (#48420)

Currently, environments can end up with higher priority than `-C` custom
config scopes and `-c` command line arguments sometimes. This shouldn't
happen -- those explicit CLI scopes should override active environments.

Up to now configuration behaved like a stack, where scopes could be only be
pushed at the top. This PR allows to assign priorities to scopes, and ensures
that scopes of lower priorities are always "below" scopes of higher priorities.

When scopes have the same priority, what matters is the insertion order.

Modifications:
- [x] Add a mapping that iterates over keys according to priorities set when
      adding the key/value pair
- [x] Use that mapping to allow assigning priorities to configuration scopes
- [x] Assign different priorities for different kind of scopes, to fix a bug, and
      add a regression test
- [x] Simplify `Configuration` constructor
- [x] Remove `Configuration.pop_scope`
- [x] Remove `unify:false` from custom `-C` scope in pipelines

On the last modification: on `develop`, pipelines are relying on the environment
being able to override `-C` scopes, which is a bug. After this fix, we need to be
explicit about the unification strategy in each stack, and remove the blanket
`unify:false` from the highest priority scope

Signed-off-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>

share/spack/gitlab/cloud_pipelines/configs/concretizer.yaml

@@ -1,3 +1,2 @@
 concretizer:
   reuse: false
-  unify: false

share/spack/gitlab/cloud_pipelines/stacks/aws-pcluster-neoverse_v1/spack.yaml

@@ -1,5 +1,8 @@
 spack:
   view: false
+  concretizer:
+    unify: false
+
   definitions:
   - apps:
     - gromacs

share/spack/gitlab/cloud_pipelines/stacks/aws-pcluster-x86_64_v4/spack.yaml

@@ -1,5 +1,8 @@
 spack:
   view: false
+  concretizer:
+    unify: false
+
   definitions:
   - apps:
     - gromacs %oneapi

share/spack/gitlab/cloud_pipelines/stacks/bootstrap-aarch64-darwin/spack.yaml

@@ -1,6 +1,9 @@
 spack:
   view: false
 
+  concretizer:
+    unify: false
+
   packages:
     all:
       require: target=aarch64

share/spack/gitlab/cloud_pipelines/stacks/bootstrap-x86_64-linux-gnu/spack.yaml

@@ -1,5 +1,7 @@
 spack:
   view: false
+  concretizer:
+    unify: false
 
   packages:
     all:

share/spack/gitlab/cloud_pipelines/stacks/build_systems/spack.yaml

@@ -1,5 +1,7 @@
 spack:
   view: false
+  concretizer:
+    unify: false
   packages:
     all:
       require: target=x86_64_v3

share/spack/gitlab/cloud_pipelines/stacks/ml-darwin-aarch64-mps/spack.yaml

@@ -1,5 +1,7 @@
 spack:
   view: false
+  concretizer:
+    unify: false
 
   packages:
     all:

share/spack/gitlab/cloud_pipelines/stacks/ml-linux-aarch64-cpu/spack.yaml

@@ -1,5 +1,7 @@
 spack:
   view: false
+  concretizer:
+    unify: false
   packages:
     all:
       require:

share/spack/gitlab/cloud_pipelines/stacks/ml-linux-aarch64-cuda/spack.yaml

@@ -1,5 +1,7 @@
 spack:
   view: false
+  concretizer:
+    unify: false
   packages:
     all:
       require:

share/spack/gitlab/cloud_pipelines/stacks/ml-linux-x86_64-cpu/spack.yaml

@@ -1,5 +1,7 @@
 spack:
   view: false
+  concretizer:
+    unify: false
   packages:
     all:
       require:

share/spack/gitlab/cloud_pipelines/stacks/ml-linux-x86_64-cuda/spack.yaml

@@ -1,5 +1,7 @@
 spack:
   view: false
+  concretizer:
+    unify: false
   packages:
     all:
       require:

share/spack/gitlab/cloud_pipelines/stacks/ml-linux-x86_64-rocm/spack.yaml

@@ -1,5 +1,7 @@
 spack:
   view: false
+  concretizer:
+    unify: false
   packages:
     all:
       require:

share/spack/gitlab/cloud_pipelines/stacks/radiuss-aws-aarch64/spack.yaml

@@ -1,5 +1,7 @@
 spack:
   view: false
+  concretizer:
+    unify: false
   packages:
     all:
       providers:

share/spack/gitlab/cloud_pipelines/stacks/radiuss-aws/spack.yaml

@@ -1,5 +1,7 @@
 spack:
   view: false
+  concretizer:
+    unify: false
   packages:
     all:
       providers:

share/spack/gitlab/cloud_pipelines/stacks/radiuss/spack.yaml

@@ -1,5 +1,7 @@
 spack:
   view: false
+  concretizer:
+    unify: false
   packages:
     all:
       require: target=x86_64_v3

share/spack/gitlab/cloud_pipelines/stacks/tutorial/spack.yaml

@@ -1,5 +1,7 @@
 spack:
   view: false
+  concretizer:
+    unify: false
   packages:
     all:
       require: target=x86_64_v3

share/spack/gitlab/cloud_pipelines/stacks/windows-vis/spack.yaml

@@ -5,6 +5,8 @@
 
 spack:
   view: false
+  concretizer:
+    unify: false
   specs:
   - vtk~mpi