Add comment about how package identity is determined

Author: mattt

Sources/PackageGraph/PackageModel+Extensions.swift

@@ -15,8 +15,16 @@ extension PackageDependencyDescription {
     /// Create the package reference object for the dependency.
     public func createPackageRef(config: SwiftPMConfig) -> PackageReference {
         let effectiveURL = config.mirroredURL(forURL: url)
+
+        // FIXME: The identity of a package dependency is currently based on
+        //        the explicit name provided in the package manifest, if provided,
+        //        falling back on a name computed from its effective URL.
+        //        We should instead use the declared URL of a package dependency as its identity,
+        //        as it will be necessary for supporting package registries.
+        let identity = explicitName?.lowercased() ?? PackageReference.computeIdentity(packageURL: effectiveURL)
+        
         return PackageReference(
-            identity: explicitName?.lowercased() ?? PackageReference.computeIdentity(packageURL: effectiveURL),
+            identity: identity,
             path: effectiveURL,
             kind: requirement == .localPackage ? .local : .remote
         )