Rework the Destination filesystem to make --renderStaticToDisk work

See #9626

common/paths/path.go

@@ -63,6 +63,15 @@ func (filepathBridge) Separator() string {
 
 var fpb filepathBridge
 
+// AbsPathify creates an absolute path if given a working dir and a relative path.
+// If already absolute, the path is just cleaned.
+func AbsPathify(workingDir, inPath string) string {
+	if filepath.IsAbs(inPath) {
+		return filepath.Clean(inPath)
+	}
+	return filepath.Join(workingDir, inPath)
+}
+
 // MakeTitle converts the path given to a suitable title, trimming whitespace
 // and replacing hyphens with whitespace.
 func MakeTitle(inpath string) string {