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/putio/object.go

@@ -125,7 +125,7 @@ func (o *Object) setMetadataFromEntry(info putio.File) error {
 // Reads the entry for a file from putio
 func (o *Object) readEntry(ctx context.Context) (f *putio.File, err error) {
 	// defer log.Trace(o, "")("f=%+v, err=%v", f, &err)
-	leaf, directoryID, err := o.fs.dirCache.FindRootAndPath(ctx, o.remote, false)
+	leaf, directoryID, err := o.fs.dirCache.FindPath(ctx, o.remote, false)
 	if err != nil {
 		if err == fs.ErrorDirNotFound {
 			return nil, fs.ErrorObjectNotFound