Bug report

What is the current behavior?

acorn's ChainExpression is not correctly parsed. Variables are not correctly resolved to imports when optional chaining is used.

import test from './test';

console.log(test.a)
console.log(test?.a)

compiles to

/* harmony import */ var _test__WEBPACK_IMPORTED_MODULE_0__ = __webpack_require__(/*! ./test */ "./src/test.js");


console.log(_test__WEBPACK_IMPORTED_MODULE_0__.default.a)
console.log(test?.a)

If the current behavior is a bug, please provide the steps to reproduce.

https://github.com/SunriseFox/webpack-bug-optional-chaining

yarn && yarn start

What is the expected behavior?

should be

/* harmony import */ var _test__WEBPACK_IMPORTED_MODULE_0__ = __webpack_require__(/*! ./test */ "./src/test.js");


console.log(_test__WEBPACK_IMPORTED_MODULE_0__.default.a)
console.log(_test__WEBPACK_IMPORTED_MODULE_0__.default?.a)

Other relevant information:
webpack version: 5.0.0-beta.22
Node.js version: v12.16.3
Operating System: MacOS 10.15.5
Additional tools: none