`import` shouldn’t have a `default` field

[wooorm]

Subject of the issue

Currently, both the import and export node have a default field. There is code to set the default field to true or false on export nodes, but the current code never sets import.default too true. So why does it exist?

Steps to reproduce

mdx/packages/mdx/index.js

Line 31 in be4877e

default: isExportDefault(subvalue),