fix: Avoid aliasing known CTEs

[snapiri]

I now also add the Common Table Expression (CTE) names to the known tables, so they will not be re-added (thus cross-joined with self).

The following snippet (from the issue) was used to validate the change:

    from sqlalchemy import create_engine, MetaData, select, func as F, Table, Column, Integer, ARRAY

    metadata = MetaData()

    table = Table(
        "table1",
        metadata,
        Column("foo", Integer),
        Column("bars", ARRAY(Integer))
    )
    engine = create_engine("bigquery://", future=True)

    selectable = sql.select(table.c).select_from(table).cte('cte')
    q = sql.select([selectable.c.foo, sql.func.unnest(selectable.c.bars).column_valued('unnest_bars')]).select_from(selectable)
    print(q.compile(engine))

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Related #368

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[tswast]

Kokoro failure looks like #875

We might need to address that before we can merge.

[tswast]

Any possibility you could add a test that was failing before this change? Or at least provide a code example so that one of the package maintainers could add such a test.

[tswast]

https://github.com/googleapis/python-bigquery-sqlalchemy/blob/main/tests/system/test_sqlalchemy_bigquery.py looks like a pretty natural spot for such a test.

[snapiri]

Test added