Do not rewrite main/module if no exports are present when bundling packages

[sndrs]

What are you changing?

#1122 added the ability to define multiple exports in packages, but it broke the build of packages which are not bundled by rollup, and removed main and module fields from their package.jsons. This fixes that.

Note

There is some useful stuff in #114 that it would be good to bring across (esp breaking up the executor into separate compile/pack executors) that may help this, so this is just a fix to get it working

Why?

it's better when our packages work

fixes #1165

[changeset-bot]

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Latest commit: 37ad757

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[github-actions]

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