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_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)
 		}
 	}
 }