Decl has a package access level but no -package-name was passed

I'm trying to implement newly added access specifier package

public class PackageDemoLibrary {    public init() {}    public var publicVariable = ""    package var packageVariable = ""}

but it's saying Decl has a package access level but no -package-name was passed.

As per documentation: A new flag -package-name is passed down to a commandline invocation, as follows. Read more here

swiftc -module-name Engine -package-name gamePkg ...swiftc -module-name Game -package-name gamePkg ...swiftc -module-name App -package-name appPkg ...

I'm not sure how can I add -package name in to my SPM Package.

Anyone know how can I get rid of this error