Update container copy to use FilePath (#1580)

- Replace `URL` with `FilePath` in `container copy` (#1557).
  In addition, make `copyIn`/`copyOut` API to use `String` for path as we
  need to preserve the trailing slash to the `LinuxContainer.copy`---i.e.,
  this trailing slash is used in `LinuxContainer.copy` to determine copy
  behavior.

Package.swift

@@ -113,6 +113,7 @@ let package = Package(
                 "ContainerRuntimeClient",
                 "ContainerRuntimeLinuxClient",
                 "ContainerVersion",
+                .product(name: "SystemPackage", package: "swift-system"),
                 "ContainerXPC",
                 "TerminalProgress",
                 "Yams",