Enhancing a theme component can only be done once

## 🐛 Bug Report

If more than one plugin enhances a theme component (https://docusaurus.io/docs/using-themes/#for-plugin-authors), such as `docusaurus-theme-live-codeblock`, subsequent enhancers past the first will receive `undefined` for the imported component to enhance from `@theme-init`

### Have you read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/master/CONTRIBUTING.md#reporting-new-issues)?

Yes

## To Reproduce

1. Install the `@docusaurus/theme-live-codeblock` package
1. Activate it _twice_ in docusaurus.config.js (to simulate more than one plugin enhancing `CodeBlock`):
```
themes: [
["@docusaurus/theme-live-codeblock", { id: "2" }],
["@docusaurus/theme-live-codeblock", { id: "1" }]
],
```

1. `docusaurus start`
1. Receive errors in the console. This is caused by [this import](https://github.com/facebook/docusaurus/blob/5534affdb9c3557c33ab662487cea325cb1ac026/packages/docusaurus-theme-live-codeblock/src/theme/CodeBlock/index.js#L11) returning undefined for the second theme definition:
```
index.js?a908:1 Warning: React.createElement: type is invalid -- expected a string (for built-in components) or a class/function (for composite components) but got: undefined. You likely forgot to export your component from the file it's defined in, or you might have mixed up default and named imports.

Check the render method of `WrappedComponent`.
at WrappedComponent (webpack-internal:///./node_modules/@docusaurus/theme-live-codeblock/src/theme/CodeBlock/index.js:13:77)
at pre (webpack-internal:///../node_modules/@docusaurus/theme-classic/lib-next/theme/MDXComponents/index.js:12:598)
at MDXCreateElement (webpack-internal:///../node_modules/@mdx-js/react/dist/esm.js:153:30)
at wrapper (webpack-internal:///../node_modules/@mdx-js/react/dist/esm.js:148:25)
at MDXCreateElement (webpack-internal:///../node_modules/@mdx-js/react/dist/esm.js:153:30)
at MDXContent (webpack-internal:///./docs/optimize/dev/react/getting-started.mdx:12:1329)

```

## Expected behavior

You can have more than one theme enhancing a given component (i.e. the second theme instance would enhance the component enhanced by the first instance).

## Actual Behavior

Importing the component after one enhancement gives undefined.

## Your Environment

- Docusaurus version used: 2.0.0-alpha.72
- CodeSandbox + node 14, windows 10 + node 14

## Reproducible Demo

[CodeSandbox](https://codesandbox.io/s/ecstatic-black-f4421?file=/docusaurus.config.js:81-204)