Storysource: Use story parameters injected by source-loader

[shilman]

PR #7117 introduced @libetl's @storybook/source-loader, a reusable package for annotating stories with their source code. In addition to emitting an event (old behavior) every time a story is rendered, it also injects story source as story parameters.

The Storysource addon should be refactored to use the story parameters instead of the channel event. There are a few reasons:

1. There are race conditions on the channel, and parameters don't have this problem.
2. Story parameters are simpler, and we are standardizing on those for communicating between the preview & manager in many cases.
3. The event emitter code breaks when you use stories like the one below, because the modified code assumes that stories are called with a context argument, which is not the case in the following example:

import React from 'react';
import { DocsPage, DocsWrapper, DocsContent } from './DocsPage';
import * as storyStories from './Story.stories';
import * as previewStories from './Preview.stories';
import * as propsTableStories from './PropsTable/PropsTable.stories';
import * as sourceStories from './Source.stories';
import * as descriptionStories from './Description.stories';

export const empty = () => (
  <DocsPage title={null}>
    {storyStories.error()}
    {propsTableStories.error()}
    {sourceStories.sourceUnavailable()}
  </DocsPage>
);

On top of all that #7117 introduced a bug where the Storysource addon no longer renders the story source once source-loader has been activated. Please fix this problem, either in source-loader or storysource (I haven't debugged the issue)

[shilman]

It turns out that there is more source-loader follow up work. Filing a separate issue that should be addressed before this one: #7204

[shilman]

Egads!! I just released https://github.com/storybookjs/storybook/releases/tag/v5.2.0-alpha.38 containing PR #7272 that references this issue. Upgrade today to try it out!

You can find this prerelease on the @next NPM tag.

Closing this issue. Please re-open if you think there's still more to do.