* Allow redeclaring CommonJS alias with initial undefined
This allows us to read our own output, plus the times when people
manually write exactly the same pattern.
Fixes#40555
* Use control flow to type commonjs exports
1. Could probably use a *lot* more tests.
2. getTypeOfAlias redoes some work from resolveAlias because it needs to
not resolve the alias completely, just to its export.
* fix lint, improve jsdoc
* Add tests, improve+fix control flow
1. Update the module.exports test to match the exports ones.
2. Add a test of evolving commonjs type.
3. Add a test of assignment as last statement.
(1) exposed a bug that required a better synthetic reference. (3)
exposed a bug that was most easily fixed by giving source files a
`endFlowNode` like functions and setting it in the binder.
* fix lint