refactor: library_private_types_in_public_api linter rule

[mfederowicz]

Summary

AVOID using library private types in public APIs.

For the purposes of this lint, a public API is considered to be any top-level or member declaration unless the declaration is library private or contained in a declaration that's library private. The following uses of types are checked:

• the return type of a function or method,
• the type of any parameter of a function or method,
• the bound of a type parameter to any function, method, class, mixin, extension's extended type, or type alias,
• the type of any top level variable or field,
• any type used in the declaration of a type alias (for example typedef F = _Private Function();), or
• any type used in the on clause of an extension or a mixin

[stale]

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