omit text from selected_columns; clear memoizations

omit text from selected_columns; clear memoizations

Fixed regression where usage of the :func:`_sql.text` construct inside the
columns clause of a :class:`_sql.Select` construct, which is better handled
by using a :func:`_sql.literal_column` construct, would nonetheless prevent
constructs like :func:`_sql.union` from working correctly. Other use cases,
such as constructing subuqeries, continue to work the same as in prior
versions where the :func:`_sql.text` construct is silently omitted from the
collection of exported columns. Also repairs similar use within the
ORM.

This adds a new internal method _all_selected_columns.  The existing
"selected_columns" collection can't store a TextClause and this never
worked, so they are omitted.  The TextClause is also not "exported",
i.e. available for SELECT from a subquery, as was already the case
in 1.3, so the "exported_columns" and "exported_columns_iterator"
accessors are where we now omit TextClause.

Fixed regression involving legacy methods such as
:meth:`_sql.Select.append_column` where internal assertions would fail.

Fixes: #6343
Fixes: #6261
Change-Id: I7c2e5b9ae5d94131c77599a020f4310dcf812bcf