Add cleanup of .g.part files (dart-lang/source_gen#362)

Towards dart-lang/source_gen#319

These never need to be served or included in the merged output directory
so they should always be cleaned by the post process builder.

source_gen/source_gen/source_gen/build.yaml

@@ -6,4 +6,9 @@ builders:
     build_extensions: {".dart": [".g.dart"]}
     auto_apply: none
     build_to: source
-    required_inputs: ['.g.part']
+    required_inputs: [".g.part"]
+    applies_builders: ["source_gen|part_cleanup"]
+post_process_builders:
+  part_cleanup:
+    import: "package:source_gen/builder.dart"
+    builder_factory: "partCleanup"

source_gen/source_gen/source_gen/lib/builder.dart

@@ -39,6 +39,9 @@ Builder combiningBuilder([BuilderOptions options]) {
   return builder;
 }
 
+PostProcessBuilder partCleanup(BuilderOptions options) =>
+    const FileDeletingBuilder(const ['.g.part']);
+
 /// A [Builder] which combines part files generated from [SharedPartBuilder].
 ///
 /// This will glob all files of the form `.*.g.part`.

source_gen/source_gen/source_gen/pubspec.yaml

@@ -8,7 +8,7 @@ environment:
 dependencies:
   analyzer: '>=0.31.0 <0.33.0'
   async: ^2.0.7
-  build: '>=0.10.0 <0.13.0'
+  build: ^0.12.4
   dart_style: '>=0.1.7 <2.0.0'
   meta: ^1.1.0
   path: ^1.3.2