[SwiftRefactor] PackageManifest: Make a couple of properties externally accessible

In it's current state `AddPackageTarget` does multiple things:

1. Updates the manifest to introduce new targets and dependencies.

2. Generates one or more auxiliary files depending on the type of
target being added (i.e. a primary file for the target and sometimes
a macro specific defintions file).

We'd like to scope the refactoring action down to manifest updates only.

The callers are going to be resposble for #2 from the list. In order to
do that `PackageTarget.sanitizedName` and `PackageTarget.Dependency.name`
have to be externally accessible (under `@_spi(PackageRefactor)` still)
to form the content of the file.

Author: xedin

Sources/SwiftRefactor/PackageManifest/AddPackageTarget.swift

@@ -295,18 +295,6 @@ public struct AddPackageTarget: ManifestEditRefactoringProvider {
   }
 }
 
-fileprivate extension PackageTarget.Dependency {
-  /// Retrieve the name of the dependency
-  var name: String {
-    switch self {
-    case .target(let name),
-      .byName(let name),
-      .product(let name, package: _):
-      return name
-    }
-  }
-}
-
 /// The array of auxiliary files that can be added by a package editing
 /// operation.
 private typealias AuxiliaryFiles = [(String, SourceFileSyntax)]
@@ -346,7 +334,8 @@ fileprivate extension PackageDependency {
   }
 }
 
-fileprivate extension PackageTarget {
+@_spi(PackageRefactor)
+public extension PackageTarget {
   var sanitizedName: String {
     name
       .mangledToC99ExtendedIdentifier()

Sources/SwiftRefactor/PackageManifest/PackageTarget.swift

@@ -51,6 +51,16 @@ public struct PackageTarget {
     case byName(name: String)
     case target(name: String)
     case product(name: String, package: String?)
+
+    /// Retrieve the name of the dependency
+    public var name: String {
+      switch self {
+      case .byName(let name),
+        .target(let name),
+        .product(let name, package: _):
+        return name
+      }
+    }
   }
 
   public init(