fix: move cac dependency to vuepress package (close: #1183)

Author: larionov

packages/@vuepress/core/package.json

@@ -37,7 +37,6 @@
     "@vuepress/shared-utils": "^1.0.0-alpha.31",
     "autoprefixer": "^7.1.2",
     "babel-loader": "^8.0.4",
-    "cac": "^6.3.9",
     "cache-loader": "^1.2.2",
     "chokidar": "^2.0.3",
     "connect-history-api-fallback": "^1.5.0",

packages/vuepress/package.json

@@ -29,7 +29,8 @@
   "homepage": "https://github.com/vuejs/vuepress#readme",
   "dependencies": {
     "@vuepress/core": "^1.0.0-alpha.31",
-    "@vuepress/theme-default": "^1.0.0-alpha.31"
+    "@vuepress/theme-default": "^1.0.0-alpha.31",
+    "cac": "^6.3.9"
   },
   "engines": {
     "node": ">=8"