Preserve used imports during filtered import cleanup

[yangfan-yf-yf]

Description

Fixes #811.

Moving a global from a package module can remove a still-used aliased import when another from import from the same package precedes it. In the reported example, moving module1_global_variable leaves proxy_module1_function() referencing utils_module after its import has been deleted, so calling it raises NameError.

I traced this to AddingVisitor: it reports a successful merge into a read-only import statement even though that statement rejects the update. Duplicate removal then deletes the other import. The visitor now accepts a read-only destination only when its existing names and aliases already cover the incoming import. New names, aliases, and star imports remain in their own statements; genuine duplicates can still be removed.

I added a move regression and import-filter tests covering distinct names, aliases, star imports, identical imports, and imports already covered by a superset or star. I also ran the original three-module example in separate Python processes before and after moving the variable: both executions now return 2 1 1.

Validation on Windows / Python 3.12.3:

• The new move regression and three distinct-import subcases fail on the unmodified implementation.
• Complete suite: 2119 passed, 12 skipped, 5 xfailed, with all six new import-filter subcases passing.
• All five configured pre-commit checks pass for the changed files.

Checklist

• I have added tests that prove my fix is effective
• I have updated CHANGELOG.md