`one_declaration_per_file`: ignore declarations less visible than a specified access level

[rgoldberg]

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Feature or Enhancement Proposal

one_declaration_per_file should accept a config key, tentatively named allow_visibility (ignore & exclude were not chosen), which makes the rule ignore declarations whose access level is less visible or equal to the specified value. e.g., the following will configure the rule to not be triggered for any extra declarations whose access level is fileprivate or private:

one_declaration_per_file:
  allow_visibility: fileprivate

This would be useful to ignore helper types that won't be visible to other code.

[SimplyDanny]

With respect to the option's name: It should be a bit more descriptive, like allow_visibility: private or similar.