error on transform-export-extensions plugin upgrade

here is a repo with failing test https://github.com/entria/graphql-dataloader-boilerplate/blob/master/.babelrc#L13

plugins: [
"transform-export-extensions"
]

will fail in babel-upgrade

it will generate:

```
plugins: [
[
"@babel/plugin-proposal-export-default-from",
"@babel/plugin-proposal-export-namespace-from",
]
]
```

instead of:

```
plugins: [
"@babel/plugin-proposal-export-default-from",
"@babel/plugin-proposal-export-namespace-from",
]
```

Start with the failing .babelrc example at the linked graphql-dataloader-boilerplate location and run babel-upgrade to reproduce the nested plugin array. Trace how transform-export-extensions is converted to the two Babel proposal plugins. Done means the generated plugins list matches the flat expected output shown in the issue.

Written by the indexing model from the issue text.