use @heroku-cli/command for completions

lib/completions.js

@@ -1,3 +1,3 @@
-const {PipelineCompletion} = require('cli-engine-heroku/lib/completions')
+const {PipelineCompletion} = require('@heroku-cli/command/lib/completions')
 
 module.exports = PipelineCompletion

package.json

@@ -7,6 +7,7 @@
     "url": "https://github.com/heroku/heroku-ci/issues"
   },
   "dependencies": {
+    "@heroku-cli/command": "^8.0.5",
     "ansi-escapes": "3.1.0",
     "bluebird": "^3.5.1",
     "cli-engine-heroku": "^5.0.4",

yarn.lock

@@ -15,7 +15,7 @@
     strip-ansi "^4.0.0"
     supports-color "^5.1.0"
 
-"@heroku-cli/command@^8.0.4":
+"@heroku-cli/command@^8.0.4", "@heroku-cli/command@^8.0.5":
   version "8.0.5"
   resolved "https://registry.yarnpkg.com/@heroku-cli/command/-/command-8.0.5.tgz#eb7e596c5a17557339f227e058fa700d4c0821e6"
   dependencies: