Add a new configuration option to select among different concretization modes

The "concretizer" section has been extended with a "duplicates:strategy"
attribute, that can take three values:

- "none": only 1 node per package
- "minimal": allow multiple nodes opf specific packages
- "full": allow full duplication for a build tool

etc/spack/defaults/concretizer.yaml

@@ -36,3 +36,9 @@ concretizer:
   # on each root spec, allowing different versions and variants of the same package in
   # an environment.
   unify: true
+  # Option to deal with possible duplicate nodes (i.e. different nodes from the same package) in the DAG.
+  duplicates:
+    # "none": allows a single node for any package in the DAG.
+    # "minimal": allows the duplication of 'build-tools' nodes only (e.g. py-setuptools, cmake etc.)
+    # "full" (experimental): allows separation of the entire build-tool stack (e.g. the entire "cmake" subDAG)
+    strategy: none