Fix build issue of LinuxSys

[Kyle-Ye]

# On root of the package
swift build --product LinuxSys # ✅ works fine
swift build --product SwiftXlib # ✅ works fine

swift build --package-path ./Sys # ❌ error: 'sys': package has unsupported layout; missing system target module map at '/tmp/AppKid/Sys/Sources/LinuxSys/module.modulemap'
swift build --package-path ./SwiftXlib # ❌ error: 'sys': package has unsupported layout; missing system target module map at '/tmp/AppKid/Sys/Sources/LinuxSys/module.modulemap'

The problem is that the target config in /Sys/Package.swift is different from Package.swift.

AppKid/Package.swift

Lines 492 to 495 in 78c9c7e

	static let linuxSys: Target = target(
	name: "LinuxSys",
	path: "Sys/Sources/LinuxSys"
	)

[smumriak]

Looks good with one change: please make the type of LinuxSys product a static librar

[smumriak]

Tho right now you should use the Package manifest from the root of the repository. I left the manifest files in individual directories with intention to split repository in future, but I am not sure when that future will come