store: use StoreEntry API instead of parsing filename in remotefilelog This is more explicit and more robust.

store: use StoreEntry API instead of parsing filename in narrow This is more explicit and more robust.

store: add a `target_id` attribute on RevlogStoreEntry This hold the "target" (file, directory, etc) of a revlog. Having this available will help a lot of code to avoid direct file path access.

store: actually tag tree manifest revlogs as manifest revlogs It turn out we have been mislabeling these for a long while. This is now fixed.

store: also gather files per revlog in `topfiles` This conclude out revlog gathering.

store: also group files by revlog in fncache version of datafiles One more step.

store: add logic to group revlog file together For now each file get its own entry, this will help stopping this, soon™. We use such gathering in the `basicstore` code.

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.