Constrain abstract specs rather than concatenating strings in the "when" context manager (#26700)

Using the Spec.constrain method doesn't work since it might
trigger a repository lookup which could break our directives
and triggers a circular import error.

To fix that we introduce a function to merge abstract anonymous
specs, based only on package names, which does not perform any
lookup in the repository.

var/spack/repos/builtin.mock/packages/with-constraint-met/package.py

@@ -17,4 +17,7 @@ class WithConstraintMet(Package):
 
     with when('@1.0'):
         depends_on('b')
-        conflicts('%gcc')
+        conflicts('%gcc', when='+foo')
+
+    with when('@0.14: ^b@:4.0'):
+        depends_on('c', when='@:15 ^b@3.8:')