Make `selected_columns` be the term used for internal columns of Select.

Select objects (such as one formatting to SELECT t.a, *, t.b AS foo, 2 FROM t assuming a table t with columns a, b, and c) have both internal columns (t.a, *, t.b, 2) and external columns (t.a, t.a, t.b, t.c, foo, 2). The former ones are now referred to as selected columns (there were sometimes referred to as column origins as well, before).