support jest 26 cjs loading of packages

[edwardfoyle]

Issue #, if available:

Description of changes:
The graphql construct package uses jest 26 which does not support loading es6 modules. Because these tests need to load the backend-output-schemas and backend-output-storage packages, we need to expose a "main" field on the package.json so that the jest cjs loader can find the package entry point.

Tested by running the gql construct test suite locally with this change

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[changeset-bot]

🦋 Changeset detected

Latest commit: ba74158

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 9 packages

Name	Type
@aws-amplify/backend-output-schemas	Patch
@aws-amplify/backend-output-storage	Patch
@aws-amplify/auth-construct-alpha	Patch
@aws-amplify/backend-auth	Patch
@aws-amplify/backend-function	Patch
@aws-amplify/backend-graphql	Patch
@aws-amplify/backend-storage	Patch
@aws-amplify/backend	Patch
@aws-amplify/storage-construct-alpha	Patch

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[sobolk]

Should we add some dummy CJS package to our repo or generate one in integration tests and add dependency on packages that have to support CJS ?

[edwardfoyle]

@sobolk yeah we might want something like that. Created a tracking issue here: #287

The lint thing catching missing dependencies is also giving me some concern. Seems like it should have caught that before this PR...