install not effective unless in package.json

[picsoung]

I had some issues while installing this plugin.

Tried to install running npm install serverless-optimizer-plugin --save
and added "plugins": ["serverless-optimizer-plugin"] on s-project.json file.

But after that running any sls command gave me an error:

sls                                                            ⏎
/usr/local/lib/node_modules/serverless/node_modules/bluebird/js/release/async.js:61
        fn = function () { throw arg; };
                           ^

ServerlessError: This plugin could not be found: serverless-optimizer-plugin
    at new ServerlessError (/usr/local/lib/node_modules/serverless/lib/Error.js:17:11)
    at Serverless._loadPlugins (/usr/local/lib/node_modules/serverless/lib/Serverless.js:298:17)
    at Serverless.loadProjectPlugins (/usr/local/lib/node_modules/serverless/lib/Serverless.js:245:10)
    at /usr/local/lib/node_modules/serverless/lib/Serverless.js:76:41
From previous event:
    at Serverless.init (/usr/local/lib/node_modules/serverless/lib/Serverless.js:72:25)
    at Object.<anonymous> (/usr/local/lib/node_modules/serverless/bin/serverless:17:12)
    at Module._compile (module.js:413:34)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Function.Module.runMain (module.js:447:10)
    at startup (node.js:139:18)
    at node.js:999:3

The only to make it work for me was to create an package.json file at project root and add this line in dependencies

"serverless-optimizer-plugin": "https://github.com/serverless/serverless-optimizer-plugin.git"

Unfortunately I have no idea what went wrong before ... so I can't really help solving this issue