fix: use dynamic publicPath for web component bundles (fix vuejs#949)

Author: yyx990803

packages/@vue/cli-service/lib/commands/build/resolveWcConfig.js

@@ -68,8 +68,10 @@ module.exports = (api, { target, entry, name }) => {
     config.output
       .filename(`${entryName}.js`)
       .chunkFilename(`${libName}.[id]${minify ? `.min` : ``}.js`)
-      // use relative publicPath so this can be deployed anywhere
-      .publicPath('./')
+      // use dynamic publicPath so this can be deployed anywhere
+      // the actual path will be determined at runtime by checking
+      // document.currentScript.src.
+      .publicPath('')
 
     // externalize Vue in case user imports it
     config

packages/@vue/cli-service/lib/commands/build/resolveWcEntry.js

@@ -33,6 +33,13 @@ import 'css-loader/lib/css-base'
 import 'vue-style-loader/lib/addStylesShadow'
 import 'vue-loader/lib/runtime/component-normalizer'
 
+;(() => {
+  let i
+  if ((i = document.currentScript) && (i = i.src.match(/(.+\\/)[^/]+\\.js$/))) {
+    __webpack_public_path__ = i[1]
+  }
+})()
+
 ${files.map(file => {
     const { camelName, kebabName } = exports.fileToComponentName(prefix, file)
     return async