This commit moves `HashedStreamReader`/`get_hash_file`/`file_md5`
into `dvc.objects.hash`.
The `get_hash_file` has been renamed to `hash_file`.
This also copies `relpath` and `is_exec` to the `dvc.fs.utils`.
Current naming for `DvcFileSystem` and `RepoFileSystem` is confusing, because
dvc is not only about data and it is much more natural to expect something
named `dvcfs` to include complete view for your whole repo (including git
files), which is what `repofs` is doing right now.
`DvcFileSystem` can provide a view not only for the repo data, but also for
external data (e.g. local external outputs). So it makes more sense to call
it a `DataFileSystem`, since it really represents the data management side
of dvc.
Saugat Pachhai (सौगात)
Ruslan Kuprieiev