[SwiftRefactor] PackageManifest: Add a public initializer to FileSystem and Registry

The initial PR that introduced these types missed that, which
made it impossible to use `PackageDependency` and `AddDependency`
refactoring externally.

Author: xedin

Sources/SwiftRefactor/PackageManifest/PackageDependency.swift

@@ -24,6 +24,10 @@ public enum PackageDependency: Sendable {
 
   public struct FileSystem: Sendable {
     public let path: String
+
+    public init(path: String) {
+      self.path = path
+    }
   }
 
   public struct SourceControl: Sendable {
@@ -48,6 +52,11 @@ public enum PackageDependency: Sendable {
     public let identity: String
     public let requirement: Requirement
 
+    public init(identity: String, requirement: Requirement) {
+      self.identity = identity
+      self.requirement = requirement
+    }
+
     /// The dependency requirement.
     public enum Requirement: Sendable {
       case exact(String)