Normalize paths within hugo

filepath was used inconsistently throughout the hugolib.  With the
introduction of source and target modules, all path are normalized to
"/".  This simplifies the processing of paths.  It does mean that
contributors need to be aware of using path/filepath in any module other
than source or target is not recommended.  The current exception is
hugolib/config.go

source/filesystem.go

@@ -27,6 +27,7 @@ func (f *Filesystem) Files() []*File {
 }
 
 func (f *Filesystem) add(name string, reader io.Reader) {
+	name = filepath.ToSlash(name)
 	f.files = append(f.files, &File{Name: name, Contents: reader})
 }