Before this change, rclone ignored the --password-command on the
rclone config setting except when decrypting an existing config file.
This change allows for offloading the password storage/generation into
external hardware key or other protected password storage.
Fixes#7859
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.
This splits config.go into ui.go for the user interface functions and
authorize.go for the implementation of `rclone authorize`.
It also moves the tests into the correct places (including one from
obscure which was in the wrong place).
If you are using rclone a library you can decide to use the rclone
config file system or not by calling
configfile.LoadConfig(ctx)
If you don't you will need to set `config.Data` to an implementation
of `config.Storage`.
Other changes
- change interface of config.FileGet to remove unused default
- remove MustValue from config.Storage interface
- change GetValue to return string or bool like elsewhere in rclone
- implement a default config file system which panics with helpful error
- implement getWithDefault to replace the removed MustValue
- don't embed goconfig.ConfigFile so we can change the methods
