refactor: Make CTransaction constructor explicit

It involves calculating two hashes, so the performance impact should be
made explicit.

Also, add the module to iwyu.

ci/test/06_script_b.sh

@@ -47,6 +47,7 @@ if [ "${RUN_TIDY}" = "true" ]; then
           " src/policy/feerate.cpp"\
           " src/policy/packages.cpp"\
           " src/policy/settings.cpp"\
+          " src/primitives/transaction.cpp"\
           " src/rpc/fees.cpp"\
           " src/rpc/signmessage.cpp"\
           " src/test/fuzz/txorphan.cpp"\