fix($core): fix windows output path issue

packages/@vuepress/core/lib/build.js

@@ -26,7 +26,7 @@ module.exports = async function build (sourceDir, cliOptions = {}) {
     return console.error(logger.error(chalk.red('Unexpected option: outDir cannot be set to the current working directory.\n'), false))
   }
   await fs.remove(outDir)
-  logger.debug('Dist directory: ' + chalk.gray(path.resolve(process.cwd(), outDir)))
+  logger.debug('Dist directory: ' + chalk.gray(require('path').resolve(process.cwd(), outDir)))
 
   let clientConfig = createClientConfig(options, cliOptions).toConfig()
   let serverConfig = createServerConfig(options, cliOptions).toConfig()

packages/@vuepress/core/lib/prepare/AppContext.js

@@ -52,8 +52,8 @@ module.exports = class AppContext {
     this.base = this.siteConfig.base || '/'
     this.themeConfig = this.siteConfig.themeConfig || {}
     this.outDir = this.siteConfig.dest
-      ? path.resolve(process.cwd(), this.siteConfig.dest)
-      : path.resolve(sourceDir, '.vuepress/dist')
+      ? require('path').resolve(process.cwd(), this.siteConfig.dest)
+      : require('path').resolve(sourceDir, '.vuepress/dist')
 
     this.pluginAPI = new PluginAPI(this)
     this.pages = [] // Array<Page>