The default (latest) version is now defined in 'generic.nix' and is the
same for the three packages.
'ocamlformat-rpc-lib' is no longer defined separately and also use the
latest version as a default.
This made ocamlformat always build with the default version of OCaml
independently from the specified version.
The library was incompatible with the other libraries when using a
version different from the default.
Move the definitions of OCamlformat packages into ocamlPackages. This
moves the package containing the tool closer to the definition of the
library.
It's now possible to build OCamlformat using a different version of
OCaml than the default.
`ocamlformat`, `ocamlPackages.ocamlformat` and
`ocaml-ng.ocamlPackages_<lastest>.ocamlformat` are all aliases.
The ocamlformat package have been split into two in version 0.25.1:
one for the library and one for the executable.
This adds both packages at the same time.
They have the same sources and very similar dependencies, for which the
definition is shared.
