store: change `_walk` return to `(filename, (type, size))` If we are to group file per revlog, having the filename as the "main key" will be useful. This change will make the following changes clearer.

store: lazily get file size on demand for the fncache case We don't have the information in the first place, so we can avoid querying the file system inconditionnaly for use case we don't needs it. This change requires the StoreFile class to be passed a vfs to retrieve the file_size if needed. In the non-fncache case, we already have the information from file system walking, so we keep it and use it.