[packages/policy] use package feerate in package validation

This allows CPFP within a package prior to submission to mempool.

src/validation.h

@@ -234,11 +234,19 @@ struct PackageMempoolAcceptResult
     * was a package-wide error (see result in m_state), m_tx_results will be empty.
     */
     std::map<const uint256, const MempoolAcceptResult> m_tx_results;
+    /** Package feerate, defined as the aggregated modified fees divided by the total virtual size
+     * of all transactions in the package.  May be unavailable if some inputs were not available or
+     * a transaction failure caused validation to terminate early. */
+    std::optional<CFeeRate> m_package_feerate;
 
     explicit PackageMempoolAcceptResult(PackageValidationState state,
                                         std::map<const uint256, const MempoolAcceptResult>&& results)
         : m_state{state}, m_tx_results(std::move(results)) {}
 
+    explicit PackageMempoolAcceptResult(PackageValidationState state, CFeeRate feerate,
+                                        std::map<const uint256, const MempoolAcceptResult>&& results)
+        : m_state{state}, m_tx_results(std::move(results)), m_package_feerate{feerate} {}
+
     /** Constructor to create a PackageMempoolAcceptResult from a single MempoolAcceptResult */
     explicit PackageMempoolAcceptResult(const uint256& wtxid, const MempoolAcceptResult& result)
         : m_tx_results{ {wtxid, result} } {}