Add package-info with validation for each package

[therepanic]

Required for automatic validation and detection of classes that violate null validation rules.

Refers to this comment: #302 (comment)

[therepanic]

Hello, @onobc. I see you recently worked on replacing @Nullable imports with identical JSpecify ones.

You need to add package-info to each package to fully utilize JSpecify. However, in your commit, you did not additionally mark @Nullable where necessary.

Because of this, the current build crashes. I can polish this in a separate commit and specify the annotation where necessary, or if you want to do it, you can use my commit as a basis.

[onobc]

Hi @therepanic , yeh I was just getting started on this now. I will take these package infos in your commit and use them in my PR (I have them locally but have not added the javadoc comment to each but you have 😄 ). I'll take it from here.

Thank you! It is so cool to have community members (in this case - you and @lobaorn-bitso) always having my back.

[onobc]

Closing in favor of #304