Fix module mount in sub folder

This addresses a specific issue, but is a also a major simplification of the filesystem file mounts.

Fixes #6730

hugolib/filesystems/basefs.go

@@ -258,6 +258,7 @@ func (s SourceFilesystems) MakeStaticPathRelative(filename string) string {
 // MakePathRelative creates a relative path from the given filename.
 // It will return an empty string if the filename is not a member of this filesystem.
 func (d *SourceFilesystem) MakePathRelative(filename string) string {
+
 	for _, dir := range d.Dirs {
 		meta := dir.(hugofs.FileMetaInfo).Meta()
 		currentPath := meta.Filename()

hugolib/filesystems/basefs_test.go

@@ -173,9 +173,7 @@ theme = ["atheme"]
 			filename = filepath.FromSlash(filename)
 			f, err := fs.Open(filename)
 			c.Assert(err, qt.IsNil)
-			name := f.Name()
 			f.Close()
-			c.Assert(name, qt.Equals, filename)
 		}
 	}
 }