NormalizePackageNames() should interpret 'exclude developer folders' to also exclude packages in ExternalActors and ExternalObjects

In NormalizePackageNames() (PackageUtilities.cpp), there is a PackageFilter parameter, which can be set to exclude developer packages (NORMALIZE_ExcludeDeveloperPackages).
The code only checks the base Developers folder, but doesn't check against _ExternalActors/Developers/ nor __ExternalObjects_/Developers/. (See code snippet below.)

const FString DeveloperFolder = IFileManager::Get().ConvertToAbsolutePathForExternalAppForRead(*FPaths::GameDevelopersDir());
//....
if (Filename.StartsWith(DeveloperFolder))
{
    PackagePathNames.RemoveAt(PackageIndex);
    //....
}

Thist was an oversight when we added ExternalActors and ExternalObjects for world partition. Removing those folders from the list of packages reported by NormalizePackageNames with NORMALIZE_ExcludeDeveloperPackages is the right thing to do.

The behavior of NORMALIZE_ExcludeNonDeveloperPackages will also need to change. I think the other EPackageNormalizationFlags will not be affected.

Find a project with a WorldPartition map in the Developers folder, or create a new WorldPartition map under Content/Developers in an existing project.

Run the DerivedDataCacheCommandlet with -ProjectOnly and WITHOUT -Dev.

Add PRAGMA_DISABLE_OPTIMIZATIONS and put a breakpoint at
DerivedDataCommandlet.cpp:568, on these lines:

		TArray<TPair<FString, FName>> PackagePaths;
		PackagePaths.Reserve(FilesInPath.Num());
		for (FString& Filename : FilesInPath)

FilesInPath will contain the external actor packages for the WorldPartition map from
_ExternalActors/Developers/ and __ExternalObjects_/Developers/.

Those packages should be excluded since -dev is not present on the commandline.