This is possible now that we no longer support go1.12 and brings
rclone into line with standard practices in the Go world.
This also removes errors.New and errors.Errorf from lib/errors and
prefers the stdlib errors package over lib/errors.
Before this change, if there was an existing file being uploaded when
a file was renamed on top of it, then both would be uploaded. This
causes a duplicate in Google Drive as both files get uploaded at the
same time. This was triggered reliably by LibreOffice saving doc
files.
This fix removes any duplicates in the upload queue on rename.
This is done by making fs.Config private and attaching it to the
context instead.
The Config should be obtained with fs.GetConfig and fs.AddConfig
should be used to get a new mutable config that can be changed.
