Add a diagnostic for implicit relative imports

[TyceHerrman]

Summary

Add detection of implicit relative imports equivalent to Basedpyright's reportImplicitRelativeImport rule.

Documentation:

https://docs.basedpyright.com/latest/benefits-over-pyright/new-diagnostic-rules/#reportimplicitrelativeimport

Existing Pyrefly handling

Pyrefly recognizes reportImplicitRelativeImport while migrating Pyright
configuration and maps it to missing-import:

46dacdd

This issue is for first-class detection of the more specific implicit-relative-
import condition rather than treating it solely as a general missing import.

Desired behavior

• Detect an unqualified import that cannot be resolved through the normal absolute import roots but resolves only by searching from the importing file’s directory upward toward the configured execution root.
• Explain that the import may fail when the file is imported as part of a package.
• Suggest either an explicit relative import or the full absolute package path.
• Do not report imports that resolve normally through configured import roots, search paths, the standard library, or installed packages.
• Provide a dedicated, independently configurable diagnostic such as implicit-relative-import.
• Define an appropriate default severity; Basedpyright enables the equivalent diagnostic as an error in its default recommended mode.