build stdlib/belt using bsb itself

[bobzhang]

As an part of #3559, we need treat stdlib/belt as an normal package

Two features needed:

• Allow user to impose dependencies manually (not captured by ocamldep)
  notably CamlinternalFormatBasics, Pervasives

  Node and Node_fs has pseudo cycle dependencies
  This is hard to solve, we have to adapt the code, in general, handwritten namespace file should depend on modules

• Allow conditional flags based on file pattern
  for example, it does not make sense to add -open Pervasives for pervasives module
  and its dependent

comments are welcome to make it more generally useful instead of specialized for building stdlib

[OlegLustenko]

I have a question from the reason-react side. Will it affect final bundle size?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.