Respect transitive imports when resolving moor converters

Might fix an error discovered in #482

moor/test/data/tables/custom_tables.g.dart

@@ -1203,7 +1203,7 @@ abstract class _$CustomTablesDb extends GeneratedDatabase {
   Selectable<MultipleResult> multiple(Expression<bool> predicate) {
     final generatedpredicate = $write(predicate, hasMultipleTables: true);
     return customSelect(
-        'SELECT d.*, "c.a" AS "nested_0.a", "c.b" AS "nested_0.b", "c.c" AS "nested_0.c" FROM with_constraints c\n INNER JOIN with_defaults d\n   ON d.a = c.a AND d.b = c.b\n WHERE ${generatedpredicate.sql}',
+        'SELECT d.*, "c"."a" AS "nested_0.a", "c"."b" AS "nested_0.b", "c"."c" AS "nested_0.c" FROM with_constraints c\n INNER JOIN with_defaults d\n   ON d.a = c.a AND d.b = c.b\n WHERE ${generatedpredicate.sql}',
         variables: [...generatedpredicate.introducedVariables],
         readsFrom: {withConstraints, withDefaults}).map(_rowToMultipleResult);
   }

moor_generator/lib/src/backends/build/preprocess_builder.dart

@@ -38,26 +38,51 @@ class PreprocessBuilder extends Builder {
     final moorFileContent = await buildStep.readAsString(input);
     final engine = SqlEngine(EngineOptions(useMoorExtensions: true));
 
-    final parsed = engine.parseMoorFile(moorFileContent);
+    final parsedInput = engine.parseMoorFile(moorFileContent);
 
-    final dartLexemes = parsed.tokens
+    final dartLexemes = parsedInput.tokens
         .whereType<InlineDartToken>()
         .map((token) => token.dartCode)
         .toList();
 
     if (dartLexemes.isEmpty) return; // nothing to do, no Dart in this moor file
 
-    final importedFiles = parsed.rootNode.allDescendants
+    // Crawl through transitive imports and find all Dart libraries
-        .whereType<ImportStatement>()
+    final seenFiles = <AssetId>{};
-        .map((stmt) => stmt.importedFile)
+    final queue = <AssetId>[input];
-        .where((import) => import.endsWith('.dart'));
+
+    while (queue.isNotEmpty) {
+      final asset = queue.removeLast();
+
+      if (!seenFiles.contains(asset)) {
+        seenFiles.add(asset);
+
+        if (asset.extension == '.moor') {
+          final parsed = asset == input
+              ? parsedInput
+              : engine.parseMoorFile(await buildStep.readAsString(asset));
+
+          parsed.rootNode.allDescendants
+              .whereType<ImportStatement>()
+              .map((stmt) => AssetId.resolve(stmt.importedFile, from: asset))
+              .where((importedId) =>
+                  !seenFiles.contains(importedId) &&
+                  !queue.contains(importedId))
+              .forEach(queue.add);
+        }
+      }
+    }
+
+    final importedDartFiles =
+        seenFiles.where((asset) => asset.extension == '.dart');
 
     // to analyze the expressions, generate a fake Dart file that declares each
     // expression in a `var`, we can then read the static type.
 
     final dartBuffer = StringBuffer();
-    for (final import in importedFiles) {
+    for (final import in importedDartFiles) {
-      dartBuffer.write('import ${asDartLiteral(import)};\n');
+      final importUri = import.uri.toString();
+      dartBuffer.write('import ${asDartLiteral(importUri)};\n');
     }
 
     for (var i = 0; i < dartLexemes.length; i++) {

moor_generator/test/backends/build/preprocess_builder_tests.dart

@@ -24,6 +24,54 @@ CREATE TABLE foo (
         'foo|converter.dart': '''
 import 'package:moor/moor.dart';
 
+class MyConverter extends TypeConverter<DateTime, int> {
+  const MyConverter();
+  
+  int mapToSql(DateTime time) => time?.millisecondsSinceEpoch;
+  
+  DateTime mapToDart(int fromSql) {
+    if (fromSql == null) return null;
+    return DateTime.fromMillisecondsSinceEpoch(fromSql);
+  }
+}
+        ''',
+      },
+      writer: writer,
+      reader: reader,
+    );
+
+    final output =
+        utf8.decode(writer.assets[AssetId.parse('foo|main.dart_in_moor')]);
+    final serialized = json.decode(output);
+
+    expect(serialized['const MyConverter()'], {
+      'type': 'interface',
+      'library': 'asset:foo/converter.dart',
+      'class_name': 'MyConverter',
+      'type_args': [],
+    });
+  });
+
+  test('finds dart files over transitive imports', () async {
+    final writer = InMemoryAssetWriter();
+    final reader = await PackageAssetReader.currentIsolate();
+
+    await testBuilder(
+      PreprocessBuilder(),
+      {
+        'foo|main.moor': '''
+import 'indirection.moor';
+
+CREATE TABLE foo (
+  id INT NOT NULL MAPPED BY `const MyConverter()`
+);
+        ''',
+        'foo|indirection.moor': ''' 
+import 'converter.dart';        
+        ''',
+        'foo|converter.dart': '''
+import 'package:moor/moor.dart';
+
 class MyConverter extends TypeConverter<DateTime, int> {
   const MyConverter();