concretizer bugfix: fix generations of conditionals for dependencies

Spack was generating the same dependency connstraints twice in the output ASP:

```
declared_dependency("abinit", "hdf5", "link")
    :- node("abinit"),
       variant_value("abinit", "mpi", "True"),
       variant_value("abinit", "mpi", "True").
```

This was because `AspFunction` was modifying itself when called.

- [x] fix `AspFunction` so that every call returns a new object

lib/spack/spack/solver/asp.py

@@ -73,13 +73,12 @@ def _id(thing):
 
 
 class AspFunction(AspObject):
-    def __init__(self, name):
+    def __init__(self, name, args=None):
         self.name = name
-        self.args = []
+        self.args = [] if args is None else args
 
     def __call__(self, *args):
-        self.args[:] = args
+        return AspFunction(self.name, args)
-        return self
 
     def __getitem___(self, *args):
         self.args[:] = args
@@ -89,6 +88,9 @@ def __str__(self):
         return "%s(%s)" % (
             self.name, ', '.join(_id(arg) for arg in self.args))
 
+    def __repr__(self):
+        return str(self)
+
 
 class AspAnd(AspObject):
     def __init__(self, *args):
@@ -394,6 +396,7 @@ def pkg_rules(self, pkg):
                             *self.spec_clauses(named_cond, body=True)
                         )
                     )
+            self.out.write('\n')
 
         # virtual preferences
         self.virtual_preferences(