[Feature request] custom `themeLayoutPath`

[tyrion-yu]

Feature request

What problem does this feature solve?

VuePress resolves theme by using path as vuepress-theme-${siteConfig.theme}/Layout.vue.
But for some security reason, some companies have their own npm registry. And the VuePress themes using in the company should only publish in the private npm registry. For no conflict with the public npm registry, the private npm packages published in the company should with a scope like @cip/vuepress-theme-security.

What does the proposed API look like?

in .vuepress/config.js

module.exports = {
  themeConfig:{
    layoutPath: `@cip/vuepress-theme-security`
  }
}

How should this be implemented in your opinion?

// resolve theme
const useDefaultTheme = (
  !siteConfig.theme &&
  !siteConfig.layoutPath &&
  !fs.existsSync(path.resolve(vuepressDir, 'theme'))
)

// prepare.js
try {
  const layoutPath = siteConfig.layoutPath? `${siteConfig.layoutPath}/Layout.vue`
    : `vuepress-theme-${siteConfig.theme}/Layout.vue`
  themeLayoutPath = require.resolve(layoutPath, {
    paths: [
      path.resolve(__dirname, '../node_modules'),
      path.resolve(sourceDir)
    ]
  })
  themePath = path.dirname(themeLayoutPath)
}

Are you willing to work on this yourself?**

I'm not sure.

[andreasvirkus]

We should be able to copy this logic from vue-cli plugin lookup. I can look into it over the coming weekend, if there's no takers beforehand