Treat stdlib as normal package

[bobzhang]

In bsconfig.json, adding a new attribute stdlib which defaults to true

The reason is that if we install bs-platform globally and do a symlink, the path resolution would still go to the absolute path which still works but may not be what we want.

The other benefit is that OCaml's stdlib is not really tailored for JS backend, this leaves more choices later.
So the compiler will always add a -nostdlib and based on user option to pick up the stdlib path.

If we treat stdlib as a normal package, make sure clean-world will not clean it and make-world will not build it...

[bobzhang]

it will affect internal package dependency handling, you need query bs-dependencies and use-stdlib (get_package_specs) to get it correct.
Again, if we have a global view of dependencies, we can deal with duplicated packages, then this will be solved in a consistent style without exceptions

[bobzhang]

Treat stdlib as a normal ocaml package requires more work(interaction with -make-world, -clean-world and duplicated packages etc), let's add a quick hack for -I, and tune better global views later

[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.