Fix tag_expression usage of frozen_children which was tag objects.

At some point, I changed tag_export.flat_dict from using strings as
keys to using actual Tag objects as keys. The tag expression handler
never got updated, so frozen_children[tagname] raised KeyError as
it was looking for a string.
I considered using tag_expression_tree.map to convert all the tree
tokens into tag objects, but when we render the tree back into text
it will say "Tag:name" instead of just "name" throughout the whole
expression, and I don't want to deal with converting those back.

etiquette/searchhelpers.py

@@ -484,6 +484,10 @@ def tag_expression_tree_builder(
     return expression_tree
 
 def tag_expression_matcher_builder(frozen_children):
+    frozen_children = {
+        (tag.name if not isinstance(tag, str) else tag): children
+        for (tag, children) in frozen_children.items()
+    }
     def match_function(photo_tags, tagname):
         '''
         Used as the `match_function` for the ExpressionTree evaluation.