hugolib, commands: Improve live-reload on directory structure changes

This issue is more visible now that we support nested sections.

This commit makes operations like pasting new content folders or deleting content folders during server watch just work.

Fixes #3570

hugolib/page.go

@@ -322,6 +322,18 @@ func (ps Pages) FindPagePosByFilePath(inPath string) int {
 	return -1
 }
 
+func (ps Pages) FindPagePosByFilePathPrefix(prefix string) int {
+	if prefix == "" {
+		return -1
+	}
+	for i, x := range ps {
+		if strings.HasPrefix(x.Source.Path(), prefix) {
+			return i
+		}
+	}
+	return -1
+}
+
 // FindPagePos Given a page, it will find the position in Pages
 // will return -1 if not found
 func (ps Pages) FindPagePos(page *Page) int {