[build] [deps] Merge `serlib` into coq-lsp

[ejgallego]

It makes a lot more sense for us to develop serlib natively here, as we need new exclusive functionalities and the submodules are not bringing us a lot.

I guess we will reverse the dependency and have sertop depend on coq-lsp serlib's, if that's still needed.

TODO:

• [x] fix genargs tests
• [ ] update Coq's CI
• [x] update SerAPI's code to avoid duplication
  • [x] see https://github.com/ejgallego/coq-lsp/pull/698
  • [x] remove sertop_loader
  • [x] remove serapi_goals [decided against]
• [ ] update jsCoq

[Alizter]

What happens to the opam package? Do you just switch the source?

[ejgallego]

What happens to the opam package? Do you just switch the source?

coq-lsp wont' depend anymore on coq-serapi

Newer coq-serapi will depend on coq-lsp, if there is a new release tho, likely the last.