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Summary:
Working on refactoring error handling in `_loadAsDir` I figured out it was oftentimes problematic to pass on the candidates out of the functions as an array, as in practice there's always a single "candidates" object passed out. Also, using an array prevented nice Flow typing and forced additional invariants to be added. So I replaced that by a return value that explicitely can be either a module, or resolution candidates. That way the semantic is more clear: we don't get any candidates if we did resolve properly, and at the same time we enforce returning candidates if we could not resolve any module.
At first I wanted to just have type `{module: TModule} | {candidate: TCandidate}`, but Flow would hit edge cases, so instead I added a field `type` that make it explicit what is the result of the resolution, and allows Flow to refine the type fully after we test that field. This allows us to remove the extraneous invariants. Also, a nice thing is that at a few places, even if the type of the candididate is different, Flow allows us to return the "resolved" object just as it is, that prevents using more memory and causing more garbage collection than necessary.
Since we're creating more objects with that solution, this will be slightly less performant than returning `Module` objects directly, but I don't think it is worth micro optimizing this at that point. If really we see this to be causing trouble later, I'd try to find solutions such as reusing a pool of objects. Ex. we could pass the result `Resolution` object as argument instead of returning a fresh one, but that would make the code less legible, more complex.
Reviewed By: davidaurelio
Differential Revision: D5111501
fbshipit-source-id: f41cdab00640124081cfdf07668169bb2d5c00be