fix(join): dereference literals in values of JoinChain

[coroa]

Description of changes

Literals as value of a JoinChain should be substituted through subsequent joins like in the example illustrated in #9561 .

A statement like
t3 = t1.inner_join(t2, "i").select("i", lit=1) creates a JoinChain looking like

JoinChain[r0]
  JoinLink[inner, r1]
    r0.i == r1.i
  values:
    i:     r0.i
    lit:   1

with one of the reference "fields" in values actually pointing to a Literal.

Adding a subsequent JoinLink to this JoinChain like by
t5 = t3.inner_join(t4, "i") fails in prepare_predicates, since the DerefMap.from_targets(..., extra=reverse) call tries to add a substitute from the new lit-field to the Literal(1) object, which does not fit the Field type, that the subs attribute of the DerefMap are expected to have.

Allowing also Literal fixes this specific example.

NOTE: I am wondering if general Values should be allowed.

In general, I am very unsure about the greater implications.

Will think about a small test.

Issues closed

• Resolves bug: SignatureValidationError from DerefMap during repeated joins #9561

[cpcloud]

I hopefully-not-too-optimistically labeled this PR for the 9.2 milestone 😬.

[coroa]

After some more thinking, I guess the alternative over at #9595 is more correct and I'd prefer to close this one.

But someone more experienced than me will have to make the correct call.

[cpcloud]

@coroa You're right, the other one is more correct because subs is intentionally restricted to Fields alone. (though both actually satisfy my complex test case).

Closing.