refactor: compile the selected_cols for the ResultNode

[chelsea-lin]

No description provided.

[sycai]

non-rhetorical question: is it possible to check alias at the BFET level?

We have something like this:

python-bigquery-dataframes/bigframes/core/nodes.py

Lines 1097 to 1113 in 1f6442e

	class AliasedRef(typing.NamedTuple):
	ref: ex.DerefOp
	id: identifiers.ColumnId
	@classmethod
	def identity(cls, id: identifiers.ColumnId) -> AliasedRef:
	return cls(ex.DerefOp(id), id)
	def remap_vars(
	self, mappings: Mapping[identifiers.ColumnId, identifiers.ColumnId]
	) -> AliasedRef:
	return AliasedRef(self.ref, mappings.get(self.id, self.id))
	def remap_refs(
	self, mappings: Mapping[identifiers.ColumnId, identifiers.ColumnId]
	) -> AliasedRef:
	return AliasedRef(ex.DerefOp(mappings.get(self.ref.id, self.ref.id)), self.id)

Checking BigFrames objects instead of SQLGlot ones might make or architecture cleaner.

This suggestion is totally optional, though.

[chelsea-lin]

That's valid concerns. I created b/419595879 to revisit the issues considering the complexity of current architecture.