Automatically enable swift-testing support when building swift-testing (#7235)

Follow-up to #7220. swift-testing itself should get swift-testing
support enabled by default, but it doesn't list itself as a dependency.
This change extends the code in that PR to also check the identities of
the packages being built (rather than just their dependencies.) If the
package being built is named "swift-testing", then support is enabled.

Author: grynspan

Sources/Commands/SwiftTestTool.swift

@@ -101,15 +101,32 @@ struct SharedOptions: ParsableArguments {
                 completion: $0
             )
         }
+
+        // Is swift-testing among the dependencies of the package being built?
+        // If so, enable support.
         let isEnabledByDependency = rootManifests.values.lazy
             .flatMap(\.dependencies)
             .map(\.identity)
             .map(String.init(describing:))
             .contains("swift-testing")
         if isEnabledByDependency {
             swiftTool.observabilityScope.emit(debug: "Enabling swift-testing support due to its presence as a package dependency.")
+            return true
+        }
+
+        // Is swift-testing the package being built itself (unlikely)? If so,
+        // enable support.
+        let isEnabledByName = root.packages.lazy
+            .map(PackageIdentity.init(path:))
+            .map(String.init(describing:))
+            .contains("swift-testing")
+        if isEnabledByName {
+            swiftTool.observabilityScope.emit(debug: "Enabling swift-testing support because it is a root package.")
+            return true
         }
-        return isEnabledByDependency
+
+        // Default to disabled since swift-testing is experimental (opt-in.)
+        return false
     }
 }