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

[bluwy]

Description

Fix #11114

This is a cherry-pick of #11234 which was previously reverted in #11270.

This removes an incorrect behaviour that's blocking correct exports of certain packages. This will fix the preact errors in ecosystem-ci and will bring an error in Histoire's CI, which we will investigate later on.

Additional context

More context in #11259 (comment)

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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