fix(metro-resolver): Skip bare ext file resolution on ESM module resolution

[kitten]

Summary

Note

Opening this mostly to ask whether this is intended behaviour, so it's potentially not as polished/finalized as other PRs. Marking from draft as open to run tests though

In CommonJS, it's normal for metro-resolver to probe node_modules/[moduleName][ext] for implementations. For example, for invariant, we'd probe node_modules/invariant.js, node_modules/invariant.web.js, etc first, when falling back to file resolution.

However, when resolving for an ESM import, this is unwanted, out-of-spec and redundant, and we can skip this resolution, since ESM expects a full node_modules structure rather than the legacy behaviour.

While this typically doesn't matter since files in node_modules have become increasingly rare, it can be used to skip redundant file resolution for ESM in node_modules.

The legacy behaviour still has to apply to sub-paths, since sub-path imports are otherwise also disabled for sub-paths without exports conditions that replace them. However, for the root path case, which is the most common, the added lookups look redundant to me.

Arguably, if we want the sub-path extensions behaviour to still apply to ESM we could disable file resolution entirely when package.json:exports are present, and instead rely on conditions. However, this would be a "more" breaking change.

Changelog: [Fix] Prevent direct file resolution in module paths for ESM

Test plan

Unit tests (LLM-written) have been added to demonstrate the behaviour