Break the fs package up into smaller parts.

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.

cmd/purge/purge.go

@@ -2,7 +2,7 @@ package purge
 
 import (
 	"github.com/ncw/rclone/cmd"
-	"github.com/ncw/rclone/fs"
+	"github.com/ncw/rclone/fs/operations"
 	"github.com/spf13/cobra"
 )
 
@@ -22,7 +22,7 @@ you want to selectively delete files.
 		cmd.CheckArgs(1, 1, command, args)
 		fdst := cmd.NewFsDst(args)
 		cmd.Run(true, false, command, func() error {
-			return fs.Purge(fdst)
+			return operations.Purge(fdst)
 		})
 	},
 }