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.

lib/pacer/pacer.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/ncw/rclone/fs"
+	"github.com/ncw/rclone/fs/fserrors"
 )
 
 // Pacer state
@@ -339,7 +340,7 @@ func (p *Pacer) call(fn Paced, retries int) (err error) {
 		fs.Debugf("pacer", "low level retry %d/%d (error %v)", i, retries, err)
 	}
 	if retry {
-		err = fs.RetryError(err)
+		err = fserrors.RetryError(err)
 	}
 	return err
 }

lib/pacer/pacer_test.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"github.com/ncw/rclone/fs"
+	"github.com/ncw/rclone/fs/fserrors"
 	"github.com/pkg/errors"
 )
 
@@ -401,7 +402,7 @@ func Test_callRetry(t *testing.T) {
 	if err == errFoo {
 		t.Errorf("err didn't want %v got %v", errFoo, err)
 	}
-	_, ok := err.(fs.Retrier)
+	_, ok := err.(fserrors.Retrier)
 	if !ok {
 		t.Errorf("didn't return a retry error")
 	}
@@ -415,7 +416,7 @@ func TestCall(t *testing.T) {
 	if dp.called != 20 {
 		t.Errorf("called want %d got %d", 20, dp.called)
 	}
-	_, ok := err.(fs.Retrier)
+	_, ok := err.(fserrors.Retrier)
 	if !ok {
 		t.Errorf("didn't return a retry error")
 	}
@@ -429,7 +430,7 @@ func TestCallNoRetry(t *testing.T) {
 	if dp.called != 1 {
 		t.Errorf("called want %d got %d", 1, dp.called)
 	}
-	_, ok := err.(fs.Retrier)
+	_, ok := err.(fserrors.Retrier)
 	if !ok {
 		t.Errorf("didn't return a retry error")
 	}