fix: Move grouped_tables into _retrieve_tables() in bigquery_metadata_extractor

[xuan616]

Signed-off-by: xuans xuan_shen@outlook.com

Summary of Changes

It is an update for my previous PR(#421) which uses dataset/table_prefix as the key of grouped_tables to solve the table key missing issue since some tables can share the same table_prefix in different datasets. I just noticed the Set self.grouped_tables is only used for a single function _retrieve_tables() in bigquery_metadata_extractor. If so, for fixing that issue, I would like to just move grouped_tables into _retrieve_tables() which only works for a given dataset so that this set can still use table_prefix as the key. It is going to make the code change conciser.

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