Add encode function for Package.t

[shonfeder]

Adds an encode function for package data, as a prerequisite of adding an encode function for Dune_project.t,

• Towards Add encode function for Dune_project.t #4702
• Prerequisite of Make dune init proj create the dune-project file  #4367

I'm starting with a small chunk of the encoding, so I can get feedback in case I'm going about this wrong.

I could particularly use advise on where/whether I should test this: It will ultimately get tested via black box tests, but seems the encode function itself could use some testing. But I haven't found clear precedent for testing encode functions in the current tests.

[rgrinberg]

I could particularly use advise on where/whether I should test this: It will ultimately get tested via black box tests, but seems the encode function itself could use some testing. But I haven't found clear precedent for testing encode functions in the current tests.

I don't think tests are necessary specifically for this function. As you said, the blackbox tests will ultimately pick up the errors.

[shonfeder]

Ok. Sounds good. I suspect there may be a subtle error or two, but I'll pick those up shortly. Thanks for the review.