dircache: simplify interface, fix corner cases and apply to backends

Dircache was changed to:

- Remove special cases for the root directory
- Remove Fatal errors
- Call FindRoot on behalf of the user wherever possible
- Bring up to modern Go standards

Backends were changed to:

- Remove calls to FindRoot
- Change calls to FindRootAndPath to FindPath
- Don't make special cases for the root

This fixes several corner cases, for example removing a non existent
directory if FindRoot hasn't been called.

backend/fichier/fichier.go

@@ -264,7 +264,7 @@ func (f *Fs) List(ctx context.Context, dir string) (entries fs.DirEntries, err e
 // NewObject finds the Object at remote.  If it can't be found
 // it returns the error ErrorObjectNotFound.
 func (f *Fs) NewObject(ctx context.Context, remote string) (fs.Object, error) {
-	leaf, directoryID, err := f.dirCache.FindRootAndPath(ctx, remote, false)
+	leaf, directoryID, err := f.dirCache.FindPath(ctx, remote, false)
 	if err != nil {
 		if err == fs.ErrorDirNotFound {
 			return nil, fs.ErrorObjectNotFound
@@ -334,7 +334,7 @@ func (f *Fs) putUnchecked(ctx context.Context, in io.Reader, remote string, size
 		return nil, err
 	}
 
-	leaf, directoryID, err := f.dirCache.FindRootAndPath(ctx, remote, true)
+	leaf, directoryID, err := f.dirCache.FindPath(ctx, remote, true)
 	if err != nil {
 		return nil, err
 	}
@@ -389,13 +389,7 @@ func (f *Fs) PutUnchecked(ctx context.Context, in io.Reader, src fs.ObjectInfo,
 //
 // Shouldn't return an error if it already exists
 func (f *Fs) Mkdir(ctx context.Context, dir string) error {
-	err := f.dirCache.FindRoot(ctx, true)
-	if err != nil {
-		return err
-	}
-	if dir != "" {
-		_, err = f.dirCache.FindDir(ctx, dir, true)
-	}
+	_, err := f.dirCache.FindDir(ctx, dir, true)
 	return err
 }
 
@@ -403,11 +397,6 @@ func (f *Fs) Mkdir(ctx context.Context, dir string) error {
 //
 // Return an error if it doesn't exist or isn't empty
 func (f *Fs) Rmdir(ctx context.Context, dir string) error {
-	err := f.dirCache.FindRoot(ctx, false)
-	if err != nil {
-		return err
-	}
-
 	directoryID, err := f.dirCache.FindDir(ctx, dir, false)
 	if err != nil {
 		return err