Hoist initial assignment to exported names in cjs to they are not blocked by bindings made by __exportStar

[weswigham]

We need this because now that __createBinding is used by __exportStar to create live-updating getters, if you had input like

export * from "mod";
export const nameFromMod = 0;

we'd output

__exportStar(exports, require("mod"));
exports.nameFromMod = 0;

the assignment will throw because the __exportStar made a getter which can't be overridden with a simple assignment. By emitting

exports.nameFromMod = void 0;
__exportStar(exports, require("mod"));
exports.nameFromMod = 0;

we prevent __exportStar from overwriting nameFromMod with a getter (so the assignment succeeds).

This also neatly ensures all export names are available to reexport when resolving within a circular module graph.

[rbuckton]

May need to discuss in design meeting.

[fightingcat]

Missed the case of exported namespace (export * as foo from 'foo').

[RobertLowe]

Curious, this violates no-void https://eslint.org/docs/2.0.0/rules/no-void