Add test for cross-module usages

test/test_api/import_tree_for_usages/__init__.py

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+"""
+An import tree, for testing usages.
+"""
+

test/test_api/import_tree_for_usages/a.py

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+from . import b
+
+def foo():
+    b.bar()

test/test_api/import_tree_for_usages/b.py

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+def bar():
+    pass