Use NODE_PATH on module resolution

[zetavg]

Search the NODE_PATH environment variable for modules if they are not found elsewhere, just like Node.js.

Note: Currently BuckleScript doesn't support $NODE_PATH either, it checks "$npm_config_prefix/lib/node_modules" instead (which I don't know the reason of doing so). Maybe we should support that too/instead.

[jaredly]

I'm wary of behavior that doesn't match bucklescript's resolution -- would this even be useful if bucklescript won't compile the module?

[jaredly]

I think this is relevant again, now that bucklescript has added support for NODE_PATH rescript-lang/rescript#3708
@zetavg do you have an example repo/setup I can use to verify this PR?