[pyrefly] Diagnose directory-relative imports

[WilliamK112]

Summary

Add a dedicated directory-relative-import diagnostic when an unqualified import resolves only through FallbackSearchPath::DirectoryRelative.

The diagnostic is a non-fatal warning by default, so the module continues to resolve while users get guidance that first emphasizes missing or misconfigured search paths and site package paths, then explicit relative or fully qualified imports. BasedPyright reportImplicitRelativeImport migration keeps the existing missing-import mapping and additionally configures the dedicated error kind.

The branch is merged with current upstream main, uses the current FindImportOptions API, and covers directory-relative fallback, explicit fallback, configured search-path precedence, end-to-end diagnostic reporting, configuration migration, generated presets, and documentation.

Fixes #4174

Test Plan

• cargo fmt --all -- --check
• cargo test -p pyrefly_config test_implicit_relative_import_maps_to_import_error_kinds
• cargo test -p pyrefly_config test_doc_headers
• cargo test -p pyrefly_config test_doc_severities
• cargo test -p pyrefly_config test_error_presets_json
• cargo test -p pyrefly test_directory_relative_import_emits_dedicated_diagnostic
• cargo test -p pyrefly test_directory_relative_fallback_reports_directory_relative_import
• cargo test -p pyrefly test_explicit_fallback_does_not_report_directory_relative_import
• cargo test -p pyrefly test_absolute_search_path_precedes_directory_relative_fallback
• python3 test.py --no-test --no-tensor-shapes --no-conformance --no-jsonschema

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D119718772. (Because this pull request was imported automatically, there will not be any future comments.)

[kinto0]

Review automatically exported from Phabricator review in Meta.

[kinto0]

looks great to me, but I wonder if this should default to "ignore" instead of "warn". cc @connernilsen for opinions

[connernilsen]

Review automatically exported from Phabricator review in Meta.

[WilliamK112]

Addressed both review threads and rebased the implementation onto current upstream main:

• Renamed the diagnostic to directory-relative-import / DirectoryRelativeImport so it does not sound like Python relative-import syntax.
• Reworded the diagnostic and docs to prioritize a missing or misconfigured search path or site package path.
• Kept reportImplicitRelativeImport mapped to missing-import and added the dedicated diagnostic mapping.
• Updated the resolver tests to the current FindImportOptions API and verified that the diagnostic is attached only to the directory-relative fallback, not explicit fallback or configured search-path resolution.
• Updated generated presets and migration documentation.

All targeted tests, documentation consistency tests, formatting, Clippy, and extension tests pass. I also ran the full 9,078-test library suite: 9,072 passed and 3 were ignored. The remaining 3 host-sensitive LSP tests fail locally because interpreter discovery selects /opt/anaconda3; that interpreter has whatthepatch installed and bypasses the synthetic project venv expectations. The pushed change does not touch interpreter discovery or those LSP tests, so CI on the clean runners should be authoritative.

Ready for another look when convenient.

[kinto0]

thanks! I think the last remaining item is to default to "warn" instead of "ignore"

[WilliamK112]

@kinto0 Addressed the last item on head 091db227b: unconfigured projects using the basic preset now report directory-relative-import as a warning instead of suppressing it. I also regenerated scripts/error_presets.json and documented the Basic warning.

Validation:

• cargo test -p pyrefly_config test_preset
• cargo test -p pyrefly_config test_error_presets_json
• cargo test test_directory_relative_import_emits_dedicated_diagnostic
• python3 test.py --no-test --no-tensor-shapes --no-conformance --no-jsonschema --no-extension

All passed. Ready for another look when convenient.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅