Allow tuning max_dupes for build dependencies (#48948)

Up to now, Spack was allowing all build-tools that
may appear in the DAG to have 2 max_dupes.

This is not needed in practice for most of them,
and adding them out of caution just increases
grounding and concretization time.

This PR makes the value of max_dupes configurable
per package, and sets only a few known packages to
2 max_dupes by default.

In case user needs different values, they can
tune the configuration for their use case.

etc/spack/defaults/concretizer.yaml

@@ -43,6 +43,22 @@ concretizer:
     # (e.g. py-setuptools, cmake etc.)
     # "full" (experimental): allows separation of the entire build-tool stack (e.g. the entire "cmake" subDAG)
     strategy: minimal
+    # Maximum number of duplicates in a DAG, when using a strategy that allows duplicates. "default" is the
+    # number used if there isn't a more specific alternative
+    max_dupes:
+      default: 1
+      # Virtuals
+      c: 2
+      cxx: 2
+      fortran: 1
+      # Regular packages
+      cmake: 2
+      gmake: 2
+      py-cython: 2
+      py-flit-core: 2
+      py-setuptools: 2
+      gcc: 2
+      llvm: 2
   # Option to specify compatibility between operating systems for reuse of compilers and packages
   # Specified as a key: [list] where the key is the os that is being targeted, and the list contains the OS's
   # it can reuse. Note this is a directional compatibility so mutual compatibility between two OS's