Remove accidental dependency on NIOFileSystem

[euanh]

swift-nio's public export of NIOFileSystem was removed in 2.86.1: apple/swift-nio#3370

NIOFileSystem was not yet supposed to be public, but _NIOFileSystem depended on it as a public import. This made it possible for containerization to see the NIOFileSystem package by accident.

Replacing the use of NIOFileSystem by _NIOFileSystem, as used elsewhere, fixes the problem.

Why does CI currently pass?

The change in swift-nio does not currently cause containerization's CI to fail because Package.resolved pins swift-nio to 2.83.0, before the change was made. New versions of upstream dependencies will not be tested until Package.resolved is explicitly updated.

When containerization is built as a dependency of a end-user project, its Package.resolved file is ignored. Instead, the dependency constraints from containerization's Package.swift file are combined with those of the project and any other library dependencies, so SwiftPM or Xcode can find a set of mutually compatible packages. This can lead to new versions of containerization's upstream dependencies being used, even though those versions have never been tested in CI.

The build failure can be demonstrated by creating a new package which depends on containerization but does not constrain package versions:

    % swift package init --type executable
    Creating executable package: test
    Creating Package.swift
    Creating Sources
    Creating Sources/test/test.swift
    % cat > Package.swift <<EOF
    heredoc> // swift-tools-version: 6.2
    // The swift-tools-version declares the minimum version of Swift required to build this package.

    import PackageDescription

    let package = Package(
        name: "test",
        platforms: [
            .macOS(.v26),
        ],
        dependencies: [
            .package(url: "https://github.com/apple/containerization", from: "0.7.2"),

        ],
        targets: [
            // Targets are the basic building blocks of a package, defining a module or a test suite.
            // Targets can depend on other targets in this package and products from dependencies.
            .executableTarget(
                name: "test",
                dependencies: [
                    .product(name: "Containerization", package: "containerization"),
                ]
            ),
        ]
    )
    EOF
    % swift build
    ...
    /private/tmp/test/.build/checkouts/containerization/Sources/ContainerizationOCI/Client/RegistryClient+Fetch.swift:25:8: error: no such module 'NIOFileSystem'
     23 |
     24 | #if os(macOS)
     25 | import NIOFileSystem
        |        `- error: no such module 'NIOFileSystem'
     26 | #endif
     27 |

[crosbymichael]

@euanh You will need to sign your commit.