Use implementation only import of Foundation in PackagePlugin, as has recently been done for manifests, and remove an extraneous leftover import (#3408)

Author: abertelrud

Sources/PackagePlugin/ImplementationDetails.swift

@@ -8,7 +8,7 @@
  See http://swift.org/CONTRIBUTORS.txt for Swift project authors
  */
 
-import Foundation
+@_implementationOnly import Foundation
 
 // The way in which SwiftPM communicates with the package plugin is an im-
 // plementation detail, but the way it currently works is that the plugin

Sources/PackagePlugin/PublicAPI/Path.swift

@@ -8,8 +8,6 @@
  See http://swift.org/CONTRIBUTORS.txt for Swift project authors
  */
 
-import Foundation
-
 /// A simple representation of a path in the file system.
 public struct Path {
     private let _string: String