ASP-based solver: variants set from cli are considered as defaults (#23542)

Variants explicitly set in an abstract root spec are considered
as defaults for the package they refer to, and they override
what is in packages.yaml and in package.py. This is relevant
only for multi-valued variants, where a constraint may extend
an already default value.

var/spack/repos/builtin.mock/packages/multivalue-variant/package.py

@@ -29,6 +29,11 @@ class MultivalueVariant(Package):
         multi=False
     )
 
+    variant(
+        'libs', default='shared', values=('shared', 'static'), multi=True,
+        description='Type of libraries to install'
+    )
+
     depends_on('mpi')
     depends_on('callpath')
     depends_on('a')