Add support for mixed ESM and CJS via mixedModules flag

[styfle]

This adds support for mixing ESM and CJS so that we have feature parity with ncc, which is basically webpack.

This will fix all the issues that were reported in the latest release:

• https://spectrum.chat/zeit/general/now-deploys-suddenly-not-working~a052cb8f-e95e-4eef-9e81-5e799d1e5ef4
• https://spectrum.chat/zeit/now/did-you-forget-to-add-it-to-dependencies-in-package-json~6b370740-866c-4404-bdcc-cc51a3bc9aec
• https://spectrum.chat/zeit/now/cannot-build-ts-api-with-now-node-anymore~a24f6eb3-201a-4ecb-8688-94b4a56d4abe
• https://github.com/zeit/now-builders/issues/791

Update

I spoke with Timer and it sounds like Next.js might want to follow the Node.js behavior. If that's the case maybe we don't want this PR to land. We need to discuss with the whole team.

Perhaps we should enable backwards compatibility through a config flag like mixedModules: true. That way user's who have broken deployments can opt-in to this behavior.

[styfle]

I deleted this test because it was contrary to the new behavior. Instead I created a new test.

[codecov-io]

Codecov Report

Merging #26 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
+ Coverage   89.88%   89.89%   +0.01%     
==========================================
  Files          10       10              
  Lines         979      980       +1     
==========================================
+ Hits          880      881       +1     
  Misses         99       99

Impacted Files	Coverage Δ
src/node-file-trace.js	90.64% <100%> (+0.06%)	⬆️
src/analyze.js	96.15% <100%> (ø)	⬆️

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[Timer]

This is completely non-standard behavior and shouldn't be allowed. The provided user code in the referenced links is not valid JavaScript.

Mixing CommonJS isn't allowed by Webpack, Node, or and standards body.

I'm not in favor of allowing this, and I'm sorry that Next.js ever supported this.
This was a side-effect of Next.js fixing a pre-Babel-v7 bug.

Also, I don't believe ncc ever supported this behavior. The only behavior ncc should've supported was a ES Modules file requiring a CommonJS file, and vice-versa. Not mixing the two within a single file.

[styfle]

@Timer I just confirmed that ncc works with the unit test I added.

cd test/unit/mixed-esm-cjs
ncc build input.js
node dist

This change is necessary to maintain backwards compatibility. I would be open to using a flag to opt-in to this non-standard behavior if you prefer.

[Timer]

Ah, this is because ncc uses webpack's legacy javascript/auto source handling method instead of javascript/esm or javascript/dynamic.

Looks like we might be stuck supporting this behavior until Node 12 is LTS with officially defined behavior.

[styfle]

@Timer I added a flag mixedModules so that we can opt-in to this "legacy" behavior.