dvc: update file utilities

[Suor]

Some file utilities live in dvc.utils.fs while some other in dvc.utils.__init__. I suggest grouping them in a single place.

Also, some of them accept Path-like objects, i.e. Path, PurePath, PathInfo, etc and some do not. This makes people guess at the call site whether they need to wrap arguments into fspath(), fspath_py35() or none. So I propose that our file utilities will always accept that, this way we will concentrate those wrapper call in one place and remove the guessing part, at least for our own functions, calling os.path.*() will still need wrapping as long as we use Python 3.5-.

• move to dvc.utils.fs (Fixed by Move fs utilities to fs.py from __init__.py #3093 )
• make Path-like objects aware (Fixed by Write test for get_inode #2673 Verify if get_mtime_and_size can take both Path objects and strings #2698 Ensure contains_symlink_up_to accept both str and Path-like objects #2740 Ensure file_md5 accept str and Path-like objects #2746 Ensure copyfile accepts str and Path-like objects #2752 Ensure makedirs accepts str and Path-like objects #2757 Ensure move accepts str and Path-like objects #2769 Ensure remove accepts str and Path-like objects #2795 Write test for tmp_fname to check if it accepts Path-like and str objects #2808 Write test for walk_files to check if it accepts Path-like and str objects #3035 Ensure relpath accepts str and Path-like objects #3037 )

[efiop]

For the record, this one is taken by @algomaster99 . Not able to assign yet, working on adding him to organization.