[validation] de-duplicate package transactions already in mempool

Summary:
As node operators are free to set their mempool policies however they
please, it's possible for package transaction(s) to already be in the
mempool. We definitely don't want to reject the entire package in that
case (as that could be a censorship vector).

We should still return the successful result to the caller, so add
another result type to MempoolAcceptResult.

This concludes backport of core#22674
https://github.com/bitcoin/bitcoin/pull/22674/commits/e12fafda2dfbbdf63f125e5af797ecfaa6488f66
https://github.com/bitcoin/bitcoin/pull/22674/commits/046e8ff264be6b888c0f9a9d822e32aa74e19b78 (partial)

Depends on D12480

Notes:

• because eCash does not have RBF, our constructor for MempoolAcceptResult (success case) would have the exact same signature as the new constructor (already-in-mempool). So in stead of adding a new constructor I made the existing one more generic to handle both cases. The rest of the codebase does not directly call this constructor, it always uses either MempoolAcceptResult::MempoolTx or MempoolAcceptResult::Success.
• We do not need to handle the same-txid-diff-wtxid case, so the code in AcceptPackage is simplified for Bitcoin ABC.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12481