Install needed folders for snapshot

[ulrikstrid]

This allows us to skip submodule checkout in git. I'm opening a companion PR on bucklescript/bucklescript as well.

[bobzhang]

LGTM, @cristianoc can you confirm it works?

[cristianoc]

@bobzhang I have abandoned this for now:
rescript-lang/rescript#3910

[ulrikstrid]

This works to build under Windows with esy now. I had to remove flexdll and use a special esy version instead, does this break the normal flow, if it does I can probably get the script to remove the current flexdll instead.

I can add the Azure Pipelines that are normally in esy-ocaml to make sure that this works @bobzhang.