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/api.go

@@ -148,11 +148,6 @@ func (f *Fs) listFolders(ctx context.Context, directoryID int) (foldersList *Fol
 }
 
 func (f *Fs) listDir(ctx context.Context, dir string) (entries fs.DirEntries, err error) {
-	err = f.dirCache.FindRoot(ctx, false)
-	if err != nil {
-		return nil, err
-	}
-
 	directoryID, err := f.dirCache.FindDir(ctx, dir, false)
 	if err != nil {
 		return nil, err