workaround Xcode 15.4 bug with swift build --arch x86_64 --arch arm64

[weissi]

Motivation:

swift build --arch x64_64 --arch arm64 on at least Xcode 15.4 fails with

.build/checkouts/swift-nio/Sources/NIOCore/NIOScheduledCallback.swift:93:18: error: '_scheduleCallback' has a package access level but no -package-name was specified: .build/checkouts/swift-nio/Sources/NIOCore/NIOScheduledCallback.swift
    package func _scheduleCallback(
                 ^

That's clearly an Xcode bug but that needs a workaround.

Modifications:

• Don't use the package modifier for now

Result:

SwiftNIO builds again.