Do not add imports if we added no type info in ATAV

In ApplyTypeAnnotationsVisitor, there are edge cases where we
might have changed the module imports even though we never wound
up applying any type annotations.

This will become even more common if we support adding
`from __future__ import annotations`, which I would like to do
soon.

To handle this, we can simply return the original tree from
`transform_module_impl` (discarding any changes from either
`self` or `AddImportsVisitor`) whenever there are no changes
in `self.annotation_counts`.

I updated the no-annotations-changed test to reflect this:
```
> python -m unittest libcst.codemod.visitors.tests.test_apply_type_annotations.TestApplyAnnotationsVisitor
...............................................
----------------------------------------------------------------------
Ran 47 tests in 2.312s

OK
```

libcst/codemod/visitors/_apply_type_annotations.py

@@ -266,7 +266,7 @@ class AnnotationCounts:
     return_annotations: int = 0
     classes_added: int = 0
 
-    def applied_changes(self):
+    def any_changes(self):
         return (
             self.global_annotations
             + self.attribute_annotations
@@ -385,7 +385,13 @@ def transform_module_impl(self, tree: cst.Module) -> cst.Module:
             self.annotations.class_definitions.update(visitor.class_definitions)
 
         tree_with_imports = AddImportsVisitor(self.context).transform_module(tree)
-        return tree_with_imports.visit(self)
+        tree_with_changes = tree_with_imports.visit(self)
+
+        # don't modify the imports if we didn't actually add any type information
+        if self.annotation_counts.any_changes():
+            return tree_with_changes
+        else:
+            return tree
 
     # smart constructors: all applied annotations happen via one of these
 

libcst/codemod/visitors/tests/test_apply_type_annotations.py

@@ -1012,11 +1012,7 @@ class C:
                     def __init__(self):
                         self.attr_will_not_be_found = None
                 """,
-                # TODO: use the annotation counts to avoid adding
-                # the import in this case.
                 """
-                from bar import X
-
                 class C:
                     def __init__(self):
                         self.attr_will_not_be_found = None
@@ -1032,7 +1028,7 @@ def test_count_annotations(
         before: str,
         after: str,
         annotation_counts: AnnotationCounts,
-        applied_changes: False,
+        any_changes: False,
     ):
         stub = self.make_fixture_data(stub)
         before = self.make_fixture_data(before)
@@ -1048,4 +1044,4 @@ def test_count_annotations(
 
         self.assertEqual(after, output_code)
         self.assertEqual(str(annotation_counts), str(visitor.annotation_counts))
-        self.assertEqual(applied_changes, visitor.annotation_counts.applied_changes())
+        self.assertEqual(any_changes, visitor.annotation_counts.any_changes())