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.

store: only access is_volatile information through the file object This make sure other code only access this information through the proper API, and it prepare for store entries to be able to agregate multiple files.

store: only access file_size information through the file object This make sure other code only access this information through the proper API, and it prepare for store entries to be able to agregate multiple files.

store: have custom init for entries class This will get useful to add special processing later in this series.

store: use specialized class for store entries We introduce two different classes for revlog and other entries. For now, we still have multiple entry for the same revlog, but we will work toward grouping the different file in a single entry in this series. Having the distinction is a step toward this goal.

store: introduce a EntryFile object to actually access file info For now a StoreEntry match a single file, but the goal is to eventually combine multiple file in a higher level Entry, so we need to introduce this distinction and use it first.

store: use a StoreEntry object instead of tuple for store files We want to make the store return more semantic information instead of a stream of file path. To achieve this, we start with adding a simple object that hold the same information as the tuple it replace, and do a simple update to the user code to fetch and use the same information. From there, we will be able to iteratively upgrade the codebase toward better objects.

store: no longer explicitly mark the splitted index as ignored Since the fncache is going to automatically recognised it as non "normal" revlog file, we don't need to explicitly make it ignored.

store: cleanup what is recognized as a revlog file The temporary file from `censor` are not relevant to the store as they are just temporary. So let `fncache` and `datafiles` forget about them.