handle namespace import helpers

[quantizor]

Fixes #315

Handle the following syntax:

import * as styled from 'styled-components'

const css = styled.css`
  background: black;
`

const GlobalStyle = styled.createGlobalStyle`
  html {
    background: black;
  }
`

const Test = styled.default.div`
  color: red;
`

const before = styled.default.div`
  color: blue;
`

styled.default.div``

export default styled.default.button``

Previously styled.default.* was not being assigned identifiers, and helpers were being assigned identifiers when they should not be.

[RobbyUitbeijerse]

@probablyup coincidentally, this update also fixed our situation where we use a cjs Rollup build (external design system) with both babel-plugin-styled-components as well as next-transpile-modules (which suddenly broke after we upgraded next.js to 11.0.1, and next-transpile-modules to 8.0.0. It resulted in this error, but is resolved after we installed the test release from #340 :-)

Thanks for the update!