fix(resolve): revert ensure exports has precedence over mainFields (#11234)

[dominikg]

This reverts commit d6eb4f2.

ecosystem-ci had a fail from histoire and bisect pointed at that commit. lets see if undoing helps.

note this is going to fail svelte tests as we have to undo an optimization based on the commit to revert.

Description

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[dominikg]

/ecosystem-ci run histoire

[vite-ecosystem-ci]

📝 Ran ecosystem CI: Open

suite	result
histoire	✅ success

[patak-cat]

Thanks for helping us go through the more stable route for the ecosystem here @dominikg