Fixes #120272

This is just a draft as the solution is quite bruteforce-y.

Some context on the bug

The issue occurs as follows, top-down (Read linked issue for context on the bug):

• A query like FROM index | EVAL x = 1 | LOOKUP l ON x | KEEP lookup_field returns no rows. Exactly the same with ENRICH
• It returns no rows because the FROM relationship is replaced by an EMPTY LocalRelationship
• The EMPTY rel comes from the EsIndex having no concrete indices
• It has no concrete indices because field-caps returns no results for that index+fields
• The fields come from fieldNames(), which must return an existing field, or _index if there's no field to query
• There's a logic in fieldNames() that says "If no fields, then use _index". It's not working, because the fields requested from LOOKUP appear in the list of fields

Potential solution

Givens:

• We don't know which fields every lookup returns, so we can't substract them from the list
• We would have to first make the field-caps for lookup
• But the field-caps requets for lookup depends on the results of fieldNames()

Which means we will probably have to reorganize the EsqlSession.analyzedPlan() code to correctly trace every field.

Current solution

This PR's solution is a simple, lazy one. It works well apparently, and I suppose it wasn't done before to try to keep the fieldNames "exact". But since enrich&lookup, it wasn't exact anymore, and we were asking for enrich/lookup index fields indistinctly.

Consider this a temporary solution until analyzedPlan() and fieldNames() get a refactor