Issue Description:

If you look at the forum thread: https://forums.swift.org/t/struct-initializer-in-extension/38063/4

There is a public type exposed from a module with no hand-written public initializers. So the compiler ends up suggesting:

// In library module
public struct PageMetadata: Codable {
    public let page: Int
    public let per: Int
    public let total: Int
}

// In client module
let pm = PageMetadata(page: 1, per: 1, total: 1)
                     --
Extra arguments at positions #​1, #​2, #​3 in call
Missing argument for parameter 'from' in call

Looking at the source code for the library leads to confusion. Where did this 'from' parameter come from? And why can't I access the typical initializer? The library's code itself seems to be using {{PageMetadata.init(page:per:total) }}just fine, so what's the issue?

Both of these have reasonable answers, but that's not clear from the diagnostic. Instead, we should say something like (straw-man example):

PageMetadata can only be initialized using 'PageMetadata.init(from:Decoder)'

I think this case should only trigger when all the public initializers of the type are synthesized.

In case we know that the initializer the user is trying to access is internal (I don't know if we serialize this information in swiftmodules/swiftinterfaces), we should say so (straw-man example):

error: PageMetadata.init(page:per:total:) cannot be accessed as it is internal to Vapor
error: PageMetadata can only be initialized using 'PageMetadata.init(from:Decoder)'