Cooker: NeverCookPackageList should store packages as LongPackageNames instead of filenames, to save memory by avoiding creation of FName for each external actor package

GetNeverCookPackageFileNames among other things adds every external actor package to NeverCookPackageList, even if they are not referenced by the rest of the cook. Since it stores the packages in the list as FNames of their normalized file path, this adds each package to the FName table. The packages would also have their filename added to the FName table by UE::Cook::FPackageData if they were referenced, but in many cases the packages are never referenced (e.g. development packages) and this is bloating the FName table for no reason. The FNames for the package names are unavoidably in the FName table because the AssetRegistry uses them, so we can store the packages as FNames to LongPackageName with no extra cost.

Change GetNeverCookPackageFileNames, and NeverCookPackageList, to store its elements as FNames to LongPackageName instead of FNames to normalized filename.

N/A