Check the bound name (alias) per import in Python RemoveImport

[knutwannheden]

Motivation

The Python RemoveImport visitor decided whether an import was still referenced using the imported name, but an aliased import such as from typing import List as L binds L, not List. Removing typing.List from a file whose code references L therefore found no references to List, deemed the import unused, and removed it — leaving the L usages dangling. This was reproducible end-to-end: running Java's ChangeType(typing.List -> list) over RPC on a file using the alias deleted the import and left broken code behind. With this fix, that flow becomes a conservative no-op until alias-aware re-adding lands on the Java ChangeType/ImportService side.

Summary

• The single statement-level _is_referenced pre-check is replaced by a per-import-entry _is_removable check that judges removability by the name the entry actually binds: its alias if present, else the imported name.
• MultiImport name lists are filtered entry by entry, so a statement mixing aliased and unaliased names (e.g. from typing import List, List as L) only drops the bindings that are unreferenced.
• The converse also holds: an aliased import is removable when its alias is unused, even if the bare imported name appears in the code — that name is a different binding.
• Module-only removal of a from X import a, b statement now also checks each entry's bound name instead of the module's last segment, so entries that are still in use are kept rather than deleted with the statement.
• _binds_other additionally excludes the entry under evaluation by identity, so the "another import already binds this name" escape hatch (kept for what ChangeType leaves behind) cannot count an import against itself; the existing module/name comparison still covers duplicate statements.
• The three near-identical MultiImport rebuild blocks are consolidated into a _prune_names helper.

Test plan

• Six new unit tests in tests/python/test_remove_import.py: aliased from-import kept while its alias is used and removed when unused, removal proceeding when only the bare imported name appears, per-entry pruning of a mixed List, List as L statement, and the aliased plain-import (import numpy as np) equivalents. Four failed before the fix; two guard the removal direction.
• python -m pytest tests/python/test_remove_import.py --timeout=60 — 16 passed.
• Full non-RPC suite python -m pytest tests/python tests/recipes --timeout=60 — 1634 passed, 6 skipped.