golang.org/x/crypto/ssh/terminal is deprecated in favor of
golang.org/x/term, see https://pkg.go.dev/golang.org/x/crypto/ssh/terminal
The latter also supports ReadPassword on solaris, so enable the
respective functionality in fs/config for solaris as well.
The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.
The new code layout is documented in CONTRIBUTING.