Fix more InternalImportsByDefault errors

[tshortli]

SwiftPM doesn't build with all toolchains when the InternalImportsByDefault upcoming feature is properly enabled.

Motivation:

With the upcoming feature InternalImportsByDefault enabled, some versions of the Swift compiler diagnose this pattern:

import protocol _Concurrency.Actor

// error: package protocol cannot refine an internal protocol
package protocol Proto: Actor {
  // ...
}

Modifications:

Address these diagnostics by using correct access levels for direct imports of _Concurrency.

Result:

SwiftPM builds with all relevant Swift compilers with InternalImportsByDefault enabled.

[tshortli]

This is a follow on to #7895.

[MaxDesiatov]

@swift-ci test windows

[MaxDesiatov]

IIRC _Concurrency is not imported implicitly when building with CMake, that's why those imports were added in the first place

[tshortli]

IIRC _Concurrency is not imported implicitly when building with CMake, that's why those imports were added in the first place

I see, I wonder if that is due to -disable-implicit-concurrency-module-import getting passed down in CMake builds for some reason? It's hard for me to imagine a reason why that flag would be needed by anything in SwiftPM, maybe it's a mistake?

[tshortli]

@swift-ci please test

[tshortli]

Yeah, the flag seems to be intentional:

> rg disable-implicit-concurrency-module-import
Utilities/bootstrap
671:        swiftpm_args += ["-Xswiftc", "-Xfrontend", "-Xswiftc", "-disable-implicit-concurrency-module-import"]

Sources/swift-bootstrap/main.swift
221:            "-Xfrontend", "-disable-implicit-concurrency-module-import",

Maybe that dates from the days before _Concurrency was available ubiquitously? In any case, I won't question it for now. I've put the imports back and tried to thread the needle differently.

[bnbarham]

Yeah, the flag seems to be intentional:

Can probably remove. It is indeed from before _Concurrency was always available. I recently removed it from the manifest build themselves, I just missed that bootstrap was also doing it.

[tshortli]

@swift-ci please test Windows