Add a temporary package product that builds our cross-import overlays. (#1197)

This PR adds a **temporary** product to our package that builds the
cross-import overlay targets needed for image attachment support. This
allows developers to try out this new code.

This product will be removed from the package in a future update.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Package.swift

@@ -84,6 +84,16 @@ let package = Package(
     )
 #endif
 
+    result += [
+      .library(
+        name: "_Testing_ExperimentalImageAttachments",
+        targets: [
+          "_Testing_AppKit",
+          "_Testing_CoreGraphics",
+        ]
+      )
+    ]
+
     result.append(
       .library(
         name: "_TestDiscovery",