Constrain protocol usage outside the module

[krilnon (Kyle Murray)]

Here's an example:

[Rejected] SE-0144: Allow Single Dollar Sign as a Valid Identifier

Proposal Link: swift-evolution/0144-allow-single-dollar-sign-as-valid-identifier.md at master · apple/swift-evolution · GitHub The review of "SE-0144: Allow Single Dollar Sign as a Valid Identifier" ran from October 14...18. The proposal has been *rejected*. The majority of the feedback on this proposal was opposed to it, because it makes the Swift language complex for a single library. While it is unfortunate that this library is out in the wild and that there are some users, it was never in…