more precise js_of_ocaml flags
more precise js_of_ocaml flags

Summary:
2 fixes here:

- splits `FINDLIB_OPTS` into `NATIVE_FINDLIB_OPTS` and `JS_FINDLIB_OPTS`, so that we can pass `JS_FINDLIB_OPTS` to the jsoo build. the difference is that the JS one does not include OS-related packages like `unix` that the .js version doesn't need nor support.
- uses `ocamlfind` to locate all of the JS stubs for the packages we're using, so that we just have to update one list (`FINDLIB_PACKAGES`)

Special thanks to hhugo for the related fix in https://github.com/facebook/flow/pull/8362 (we're landing these incrementally to get our internal builds updated)

Reviewed By: vsiles

Differential Revision: D24505008

fbshipit-source-id: 9fadb3b0819ab369728e9802e1354a512152a1c9

facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow