fix #1708 prioritise vuepress dependencies over cwd node_modules

[ColinKinloch]

Summary

Have webpack prioritise the use of vuepress dependencies over the working directories node_modules.

Fixes #1708

Currently if vuepress is used to generate docs for a project depending on the latest version of css-loader yarn vuepress dev docs serves a blank screen.

If package.json resembles this:

{
  "scripts": {
    "docs:dev": "vuepress dev docs"
  },
  "dependencies": {
    "vuepress": "^1.0.2",
    "css-loader": "^3.1.0"
  }
}

vuepress will serve a blank page with the error:

ValidationError: Invalid options object. CSS Loader has been initialised using an options object that does not match the API schema.
 - options has an unknown property 'exportOnlyLocals'. These properties are valid:

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

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

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

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

It will be more difficult to overwrite the versions of packages vuepress depends on.

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)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

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:

This will make the instruction on the front page of https://vuepress.vuejs.org/ more likely to work on an existing project.

[vue-bot]

Hey @ColinKinloch, thank you for your time and effort spent on this PR, contributions like yours help make Vue better for everyone. Cheers! 💚

[tychenjiajun]

@ColinKinloch @ulivz Is this PR introduced issue #1729 ?