Unify exports implementation across CJS and ESM

[jkrems]

Right now we have two copies of the exports logic. One in CJS and one in ESM. We should try to reuse at least some of the implementation so we can spend less energy trying to sync the two.

[jkrems]

Step 1: Unify the test suite so that the entire behavior (and the differences) are covered.

[jkrems]

@guybedford @bmeck While working on the test PR (nodejs/node#28831) I ran into the ERR_MODULE_NOT_FOUND vs. MODULE_NOT_FOUND inconsistency between ESM and CJS. Was that done on purpose or is that just an artifact of the error macros and could (should?) be fixed?

[bmeck]

It was not intentional on my behalf

[jkrems]

Created a dedicated issue in case somebody else beats me to it: #360

[jkrems]

I'm starting to lean towards closing this issue as wont-fix. I haven't found a good way to do this and as long as the tests are shared, it feels like we can live with two implementations of exports, just like we live with two implementations of resolution in general.