fix: add acorn-walk as dependency instead of relying on acorn's internal folder structure

fixes #2848

Author: haoqunjiang

packages/@vue/cli-service/lib/webpack/analyzeBundle.js

@@ -2,7 +2,7 @@
 // Modified by Guillaume Chau (Akryum)
 
 const acorn = require('acorn')
-const walk = require('acorn/dist/walk')
+const walk = require('acorn-walk')
 const mapValues = require('lodash.mapvalues')
 const transform = require('lodash.transform')
 const zlib = require('zlib')

packages/@vue/cli-service/package.json

@@ -27,6 +27,7 @@
     "@vue/preload-webpack-plugin": "^1.1.0",
     "@vue/web-component-wrapper": "^1.2.0",
     "acorn": "^6.0.2",
+    "acorn-walk": "^6.1.0",
     "address": "^1.0.3",
     "autoprefixer": "^8.6.5",
     "cache-loader": "1.2.2",

yarn.lock

@@ -2446,7 +2446,7 @@ acorn-jsx@^5.0.0:
   resolved "https://registry.yarnpkg.com/acorn-jsx/-/acorn-jsx-5.0.0.tgz#958584ddb60990c02c97c1bd9d521fce433bb101"
   integrity sha512-XkB50fn0MURDyww9+UYL3c1yLbOBz0ZFvrdYlGB8l+Ije1oSC75qAqrzSPjYQbdnQUzhlUGNKuesryAv0gxZOg==
 
-acorn-walk@^6.0.1:
+acorn-walk@^6.0.1, acorn-walk@^6.1.0:
   version "6.1.0"
   resolved "https://registry.yarnpkg.com/acorn-walk/-/acorn-walk-6.1.0.tgz#c957f4a1460da46af4a0388ce28b4c99355b0cbc"
   integrity sha512-ugTb7Lq7u4GfWSqqpwE0bGyoBZNMTok/zDBXxfEG0QM50jNlGhIWjRC1pPN7bvV1anhF+bs+/gNcRw+o55Evbg==