Harmonize rule extensions

[SimplyDanny]

This allows to reduce existential types later.

[SwiftLintBot]

	3 Warnings
⚠️	Big PR
⚠️	If this is a user-facing change, please include a CHANGELOG entry to credit yourself! You can find it at CHANGELOG.md.
⚠️	This PR may need tests.

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Here's an example of your CHANGELOG entry:

* Use consistent `internal` visibility for rule extensions.  
  [SimplyDanny](https://github.com/SimplyDanny)
  [#issue_number](https://github.com/realm/SwiftLint/issues/issue_number)

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[jpsim]

This allows to reduce existential types later.

How? On its own, I preferred keeping private implementation details of these rules private and inaccessible to the rest of the module.

[SimplyDanny]

This allows to reduce existential types later.

How? On its own, I preferred keeping private implementation details of these rules private and inaccessible to the rest of the module.

The method that creates the rewriter for a rule

func makeRewriter(file: SwiftLintFile) -> (any ViolationsSyntaxRewriter)?

returns an existential. If the rewriters are internal this can become an opaque result type:

func makeRewriter(file: SwiftLintFile) -> (some Rewriter)? {

[SimplyDanny]

Turns out, the compiler confused me. Visibility can remain private for this to work (which makes more sense admittedly). A few things can still be harmonized, though. Let me update the PR keeping the extensions private ...