Resolve traits on top level manifests explicitly (#9300)

If a package is contained in a wrapper project it will have a top level
manifest with no root packages. In this case, we weren't resolving
default traits on the dependencies within this top level manifest.

Author: plemarquand

Sources/Workspace/Workspace+Manifests.swift

@@ -594,6 +594,15 @@ extension Workspace {
         // optimization: preload first level dependencies manifest (in parallel)
         let firstLevelDependencies = try topLevelManifests.values.map { manifest in
             let parentEnabledTraits = self.enabledTraitsMap[manifest.packageIdentity]
+
+            // Resolve and flatten the list of traits on top level manifests. This handles
+            // the case where a package is being loaded in a wrapper project (not package),
+            // where there are no root packages but there are dependencies.
+            if root.packages.isEmpty {
+                let topLevelManifestTraits = try manifest.enabledTraits(using: parentEnabledTraits, nil)
+                self.enabledTraitsMap[manifest.packageIdentity] = topLevelManifestTraits
+            }
+
             return try manifest.dependencies.filter { dep in
                 let explicitlyEnabledTraits = dep.traits?.filter({
                     guard let condition = $0.condition else { return true }