fix(plugin-blog): read layoutComponents from themeAPI

[jkzing]

Summary

It seems layoutComponentMap has been refactored into themeAPI, correct the reference in plugin-blog.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
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If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

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The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

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• Related documents have been updated
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To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[ulivz]

Thanks

BTW, we're planning to refactor the blog plugin because of its limitations. here's the roadmap: https://github.com/vuejs/vuepress/projects/1.

[jkzing]

@ulivz , good to know, can't wait to see it. :)

[ulivz]

Available at 1.0.0-alpha.43.