Enhancement: split the SQL `planner` into smaller modules

[jackwener]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
The current planner file is too large, we can split it.

Like

• query.rs
• select.rs
• set_expr.rs
• ....

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[jackwener]

Maybe we can add label good-first-issue

[alamb]

I agree this would be an excellent idea

The planner code I think is the code that translates from SQL statements to LogicalPlan: https://github.com/apache/arrow-datafusion/blob/a31b44eae236920d43ddd3ef7fadf8f9abf18976/datafusion/sql/src/planner.rs

[alamb]

I agree this would be a good first issue as it will be mostly a software engineering exercise and doesn't require much datafusion internals knowledge. Thank you @jackwener