Use publishers instead of builders for prep / compile / gen

[TheLortex]

Currently, we're using Current_cache.Make (https://ocurrent.org/current/Current_cache/Make/index.html) to cache builds. However, some situations might lead into inconsistencies between the data server and the ocurrent state:

• compile package a.1.0 with voodoo version x: outputs files into p/a/1.0 and obtain the hash h-x
• compile package a.1.0 with voodoo version y: outputs files into p/a/1.0 and obtain a new hash h-y
• rollback voodoo and thus compile package a.1.0with voodoo version x: cache hit, no rebuild, returns hash h-x even if the hash of the files on the server is still h-y

Current_cache.Output (https://ocurrent.org/current/Current_cache/Output/index.html) is more appropriate to reflect what we're doing. In the previous example, the key would be a.1.0 and the value the voodoo version.

[TheLortex]

Obviously making the changes will trigger a full rebuild

[tmcgilchrist]

Related to https://github.com/ocurrent/ocaml-docs-ci/issues/121