[jnigen] Change all `com.github.dart_lang.` package prefix to `dev.dart.`

[HosseinYousefi]

It's somewhat weird that the packages have com.github eventhough they are not affiliated with the github organization.

[dcharkes]

I think we chose this because that's where the package is hosted. E.g. repository url, but that might not be the right thing to do. (Speaking as someone who almost never devs Java code.)

[HosseinYousefi]

From https://docs.oracle.com/javase/tutorial/java/package/namingpkgs.html

Companies use their reversed Internet domain name to begin their package names—for example, com.example.mypackage for a package named mypackage created by a programmer at example.com.

So I think it's a better idea to have our packages be dev.dart.jnigen.mypackage instead

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Prior art in Flutter, we use io.flutter.xyz for package names there: https://api.flutter.dev/javadoc/io/flutter/embedding/engine/FlutterEngine.html