Make kpm support three kinds of local repositories

- FlatListLocalPackageRepository: repo\{id}.{version}.nupkg
- NuGetStyleLocalPackageRepository: repo\{id}.{version}\{id}.{version}.nupkg
- LayeredLocalPackageRepository: repo\{id}\{version}\{id}.{version}.nupkg
Instead of GetFiles and GetDirectories, you'd want EnumerateFiles and EnumerateDirectories