chore: clean up SupplyChainBuilder a little

packages/cli/bin/exports_supply_chain.dart

@@ -77,24 +77,23 @@ class DescribingVisitor extends SupplyLinkVisitor {
 
 void source(
   MarketListingSnapshot marketListings,
-  SystemsCache systemsCache,
-  TradeExportCache exportCache,
+  SystemsCache systems,
+  TradeExportCache exports,
   MarketPriceSnapshot marketPrices,
   TradeSymbol tradeSymbol,
   WaypointSymbol waypointSymbol,
 ) {
   logger.info('Sourcing $tradeSymbol for $waypointSymbol');
   final action = SupplyChainBuilder(
-    marketListings,
-    systemsCache,
-    exportCache,
+    systems: systems,
+    exports: exports,
+    marketListings: marketListings,
   ).buildChainTo(tradeSymbol, waypointSymbol);
   if (action == null) {
     logger.warn('No source for $tradeSymbol for $waypointSymbol');
     return;
   }
-  final visitor = DescribingVisitor(systemsCache, marketPrices);
-  action.accept(visitor);
+  action.accept(DescribingVisitor(systems, marketPrices));
 }
 
 Future<void> command(FileSystem fs, Database db, ArgResults argResults) async {

packages/cli/lib/supply_chain.dart

@@ -46,7 +46,7 @@ abstract class SupplyLink {
   /// The trade symbol being supplied.
   final TradeSymbol tradeSymbol;
 
-  /// Does a depth-first traversal of the supply chain.
+  /// Does a depth first walk with pre-order traversal.
   void accept(SupplyLinkVisitor visitor, {int depth = 0});
 }
 
@@ -155,11 +155,11 @@ MarketListing? _nearestListingWithExport(
 /// Builds a supply chain.
 class SupplyChainBuilder {
   /// Create a new supply chain builder.
-  const SupplyChainBuilder(
-    MarketListingSnapshot marketListings,
-    SystemsCache systems,
-    TradeExportCache exports,
-  )   : _marketListings = marketListings,
+  const SupplyChainBuilder({
+    required SystemsCache systems,
+    required MarketListingSnapshot marketListings,
+    required TradeExportCache exports,
+  })  : _marketListings = marketListings,
         _systems = systems,
         _exports = exports;