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/check/check.go

@@ -2,7 +2,7 @@ package check
 
 import (
 	"github.com/ncw/rclone/cmd"
-	"github.com/ncw/rclone/fs"
+	"github.com/ncw/rclone/fs/operations"
 	"github.com/spf13/cobra"
 )
 
@@ -37,9 +37,9 @@ to check all the data.
 		fsrc, fdst := cmd.NewFsSrcDst(args)
 		cmd.Run(false, false, command, func() error {
 			if download {
-				return fs.CheckDownload(fdst, fsrc)
+				return operations.CheckDownload(fdst, fsrc)
 			}
-			return fs.Check(fdst, fsrc)
+			return operations.Check(fdst, fsrc)
 		})
 	},
 }