better module names inferred for externals

[bobzhang]

Quite urgly output

var MaterialUi$slashcolors$slashindex = require("material-ui/colors/index");

Maybe we can put some constraints on bs.module name path, like this:

external teal: colorGroup = "teal" [@@bs.val] [@@bs.module "material-ui","colors","index"]

[stale]

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