What's the purpose of this change?

There are cases that the packages are missing at ncc build time, yet presented at global scope or runtime. Without this fix, the eval("require")('dep') can succeed at runtime but the result is not exported.

There are cases that the packages are missing at ncc build time, yet presented at global scope or runtime.

Which cases? Can you add a test that fails but succeeds with this PR?

Let's say we have a project referencing an optional dependency using

try {
  foo = require('foo');
} catch {}

Package foo is not installed on build time since it's totally optional and should not be bundled into the products. So package foo will be transformed by loader runtime-notfound to eval("require")("foo") without assigning return value to module.exports. Since then the require('foo') can succeed without any error at runtime but the return value is undefined, not the dynamically required package module.

The test case test/unit/runtime-notfound/output.js is already presented in the repo, and is identical to the case above, which has been updated in the PR to reflect the correct result.

Sorry I should have been more clear. Can you add an integration test that fails with the latest ncc but succeeds with this PR?

Integration test added, PTAL :)