Would be interesting to try this approach — for each package we can produce a prefix (a path with bin/, etc/) where we would place etc/findlib.conf and link (hardlink) executables from dependencies into bin/*. That prefix can by cached by build id too.

As a side effect — we'll get env size reduction because we won't need to accumulate all packages's bin into $PATH like we do now.

Let me try that after I make findlib configs work in all envs.

This is ready for review. I'd like to wrap this up as quickly as possible and move to other related issues.

After refactoring, we now create a package-level prefix and store build config in it.

We also make sure that the prefix exists even for esy CMD invocations. this helps us with running arbitrary commands that involve findlib config, e.g esy ocamlfind list (when the root is not built).

Current use of prefix is limited to keeping findlib.conf in it, but we have plans to extend it to things like ld.conf and executable links. This will help with pollution of env.

Going to close this for now, we are going to try creating a package-specific prefix in the next version of the builder though. This concept seems sound.