Allow forced deactivation -- best effort unlinking

spack deactivate -f will unlink even if Spack thinks the package isn't enabled.
Made deactivate routines idempotent.

lib/spack/llnl/util/link_tree.py

@@ -175,6 +175,10 @@ def unmerge(self, dest_root, **kwargs):
         kwargs['order'] = 'post'
         for src, dest in traverse_tree(self._root, dest_root, **kwargs):
             if os.path.isdir(src):
+                # Skip non-existing links.
+                if not os.path.exists(dest):
+                    continue
+
                 if not os.path.isdir(dest):
                     raise ValueError("File blocks directory: %s" % dest)