Feature: Allow lmod configuration to set core specs (#16517)

Packages built with lmod core_compiler are placed in `Core`.

Other packages may belong in `Core`. For example, python may be built with a proprietary compiler for performance, but belong on the `Core` directory.

With this PR, lmod config can include a `core_specs` list. Any package that satisfies a spec in that list is placed in `Core`, regardless of its compiler or dependencies.

lib/spack/docs/module_file_support.rst

@@ -538,6 +538,8 @@ most likely via the ``+blas`` variant specification.
          lmod:
            core_compilers:
              - 'gcc@4.8'
+           core_specs:
+             - 'python'
            hierarchy:
              - 'mpi'
              - 'lapack'
@@ -547,6 +549,15 @@ most likely via the ``+blas`` variant specification.
      implementations of ``mpi`` and ``lapack``, and let LMod switch safely from one to the
      other.
 
+     All packages built with a compiler in ``core_compilers`` and all
+     packages that satisfy a spec in ``core_specs`` will be put in the
+     ``Core`` hierarchy of the lua modules.
+
+.. warning::
+  Consistency of Core packages
+   The user is responsible for maintining consistency among core packages, as ``core_specs``
+   bypasses the hierarchy that allows LMod to safely switch between coherent software stacks.
+
 .. warning::
   Deep hierarchies and ``lmod spider``
    For hierarchies that are deeper than three layers ``lmod spider`` may have some issues.