It is not meaninful to run txorphanage without a node (validation, chainstate, mempool, rpc, ...). The module is in the node library and won't be added to the kernel library. Thus, it should be moved to src/node.
Also, move DEFAULT_MAX_ORPHAN_TRANSACTIONS to node/txorphanage.h, as it logically belongs to TxOrphanage::m_orphans and not to PeerManager.