`pl.Expr.meta.has_multiple_outputs()` is broken on main

[kszlim]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

import polars as pl

struct_col = pl.col("struct")

out = struct_col.struct[0] - struct_col.struct[1]
out = out.alias("output")
print(out.meta.output_name(), out.meta.has_multiple_outputs())

Log output

output True

Issue description

It should print:

output False

Expected behavior

It should print:

output False

Installed versions

On this commit e99abdc

[coastalwhite]

I am not 100% sure how we would want to approach this. It has always been broken, and I am not actually sure what this function should return.

The implementation was always basically:

Does the expression contain something we need to expand, then true. Otherwise, `false.

It is still the same implementation; nothing changed. What changed is how we expand. Maybe we need an Expr.expand(self, *, schema) -> list[Expr].

[kszlim]

Hmm, the inconsistency between has_multiple_outputs and output_name might be something to look at, even if it's still wrong I think the prior output is better.