refactor install_tree to use projections format (#18341)

* refactor install_tree to use projections format

* Add update method for config.yaml

* add test for config update config

etc/spack/defaults/config.yaml

@@ -16,7 +16,10 @@
 config:
   # This is the path to the root of the Spack install tree.
   # You can use $spack here to refer to the root of the spack instance.
-  install_tree: $spack/opt/spack
+  install_tree:
+    root: $spack/opt/spack
+    projections:
+      all: "${ARCHITECTURE}/${COMPILERNAME}-${COMPILERVER}/${PACKAGE}-${VERSION}-${HASH}"
 
 
   # Locations where templates should be found
@@ -24,10 +27,6 @@ config:
     - $spack/share/spack/templates
 
 
-  # Default directory layout
-  install_path_scheme: "${ARCHITECTURE}/${COMPILERNAME}-${COMPILERVER}/${PACKAGE}-${VERSION}-${HASH}"
-
-
   # Locations where different types of modules should be installed.
   module_roots:
     tcl:    $spack/share/spack/modules