Use qualified aliases to simplify searching DFSchema

[jonahgao]

Which issue does this PR close?

N/A.

Rationale for this change

The following logic was introduced for single_distinct_to_groupby in #4050.
The reason was the lack of alias-with-qualifier, which caused the qualifier to be lost after alias. See comment
When searching for a qualified name in DFSchema, we must also check unqualified fields because they may have been aliased and previously had a qualifier. We can now use qualified aliases to avoid this.

datafusion/datafusion/common/src/dfschema.rs

Lines 358 to 369 in 0243ebd

	(Some(qq), None) => {
	// the original field may now be aliased with a name that matches the
	// original qualified name
	let column = Column::from_qualified_name(f.name());
	match column {
	Column {
	relation: Some(r),
	name: column_name,
	} => &r == qq && column_name == name,
	_ => false,
	}
	}

What changes are included in this PR?

Are these changes tested?

By existing tests.

Are there any user-facing changes?

No

[jonahgao]

This query will not produce an error on main branch, which is incorrect.

[alamb]

Thank you @jonahgao -- this makes sense to me. Given all the tests are passing I think it is good to go 🚀

[jonahgao]

Thanks @alamb for the review