Don't track .packages file generated by pub

c.f. https://dart.dev/guides/libraries/private-files

.gitignore

@@ -0,0 +1,4 @@
+.dart_tool/
+.packages
+build/
+pubspec.lock

.packages

@@ -1,8 +0,0 @@
-# Generated by pub on 2019-06-18 10:12:43.975273.
-collection:file:///Users/juliuscanute/.pub-cache/hosted/pub.dartlang.org/collection-1.14.11/lib/
-flutter:file:///Users/juliuscanute/Library/CrossPlatform/flutter/packages/flutter/lib/
-meta:file:///Users/juliuscanute/.pub-cache/hosted/pub.dartlang.org/meta-1.1.6/lib/
-sky_engine:file:///Users/juliuscanute/Library/CrossPlatform/flutter/bin/cache/pkg/sky_engine/lib/
-typed_data:file:///Users/juliuscanute/.pub-cache/hosted/pub.dartlang.org/typed_data-1.1.6/lib/
-vector_math:file:///Users/juliuscanute/.pub-cache/hosted/pub.dartlang.org/vector_math-2.0.8/lib/
-qr_code_scanner:lib/