add back CJS<>CJS compatability from an earlier pr

[FredKSchott]

Changes

• We thought that this may not be needed, so it had been removed before merging Simplify. cleanup, enhance snowpack internals #2707
• However, we've found a case where this is required: A CJS package exports its entire namespace as a function AND attaches properties to that function. This is something that is really hard to represent in ESM, since the module.export result is both the default function and the namespace itself.
• Add back support with a basic check for CJS<>CJS package imports to always use the default import.

Testing

• N/A (we should add a test for React v16.14, but I want to get this out first so that its not broken overnight)

Docs

• N/A

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