[pyrefly] Deduplicate global missing-import paths

[lyydsheep]

Fixes #4246

Summary

• When migrated mypy configuration enables global missing-import suppression, emit only the * wildcard.
• Preserve specific module patterns and their order when no global wildcard is present.
• Cover both global ignore_missing_imports and global follow_imports = skip mixed with specific module settings.
• The local .scratch planning files are intentionally not included in this PR.

Validation

• cargo test -p pyrefly_config migration::ignore_missing_imports — 10 passed
• cargo test -p pyrefly commands::init — 34 passed
• python3 test.py --no-test --no-tensor-shapes --no-conformance --no-jsonschema — passed; one pre-existing Clippy warning in crates/pyrefly_config/src/error_kind.rs
• cargo fmt --all -- --check and git diff --check — passed

[meta-codesync]

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