Replace 'layer' string argument to upload handlers with UploadLayer instance.

[JivanAmara]

The 'layer' argument to import handlers comes from UploadLayer.name and can be a number of different values, including a name, a path, a geoserver store:name combo. Let's normalize this to an UploadLayer instance; from that, things like a name or a path can be easily grabbed & will make for clearer code.