Document common scenario of packages incorrectly declaring they have a default export instead of `export =`

[dsherret]

denoland/deno#17058 (comment)

[kwhinnery]

Think I need to dig in for more context, but transferring for now

[hashrock]

Maybe adding the example code mentioned in the issue in migration doc or faq?

[dsherret]

Going to close this one because it's becoming less of an issue thankfully and it's kind of complicated to explain.