Update .gitignore to new `dart_tool` pub cache (dart-lang/pub_semver#31)

dart-lang/sdkdart-lang/pub_semver#32030
diff --git a/pkgs/pub_semver/.gitignore b/pkgs/pub_semver/.gitignore
index 98fed01..49ce72d 100644
--- a/pkgs/pub_semver/.gitignore
+++ b/pkgs/pub_semver/.gitignore
@@ -1,6 +1,3 @@
+.dart_tool/
 .packages
-packages
 pubspec.lock
-
-# IntelliJ project.
-.idea