[AddImportsVisitor] generate deterministic add import output by sorting the sets.

[jimmylai]

Summary

@pradeep90 reported that AddImportsVisitor generates the added import alias in random order which make codemod unit test failed randomly.

In the example code like this, the order of Any, Dict, List is not deterministic.

                """
                from typing import Any, Dict, List
                from typing import Any, List
                def foo() -> List[Any]: ...
                def goo() -> Dict[Any, Any]: ...
                """,
                """
                from typing import Any
                def foo():
                    return []
                def goo():
                    return {}
                """,
                """
                from typing import Any, Dict, List
                def foo() -> List[Any]:
                    return []
                def goo() -> Dict[Any, Any]:
                    return {}
                """,

Test Plan

Add new test case.

[codecov-io]

Codecov Report

Merging #274 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #274   +/-   ##
=======================================
  Coverage   93.95%   93.96%           
=======================================
  Files         219      219           
  Lines       21241    21245    +4     
=======================================
+ Hits        19958    19962    +4     
  Misses       1283     1283           

Impacted Files	Coverage Δ
libcst/codemod/visitors/_add_imports.py	95.23% <ø> (ø)
libcst/codemod/visitors/tests/test_add_imports.py	100.00% <100.00%> (ø)

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