refactor: represent virtual libraries via Lib_kind.t

[shonfeder]

Closes #12253

The refactor here is quite straightforward, with the exception of maintaining backwards compat with current syntax of library stanzas. To achieve this, we infer kind virtual whenever virtual_modules are specified alongside a compatible kind (normal or virtual).

Two incidental improvements are also suggested here: small fixes to the docs and cleaning up a case analysis handling ppx_* kinds.

TODO

To wrap this up, I need

• input on how to signal the right error for incompatible library specifications
• add a test case to check the error handling of invalid cases

Reviewing

Initial review by commit is probably helpful, since the commits are broken into a few tiny, distinct phases.

[shonfeder]

This should be ready for a final review pass. Thank you for the input @Alizter and @rgrinberg! It was very helpful :)

[shonfeder]

This is ready for review again :)

[shonfeder]

Thanks for the review and patient guidance!