chore: remove legacy core/findPlugins (#235)

Summary:
---------

Now that we prepare 2.0 release, we can remove it. Also added `build-clean` to `postinstall` hook to avoid publishing extras when not using lerna.


Test Plan:
----------

@grabbou

Author: thymikee

package.json

@@ -10,7 +10,7 @@
     "test": "jest",
     "lint": "eslint . --cache --report-unused-disable-directives",
     "flow-check": "flow check",
-    "postinstall": "yarn build",
+    "postinstall": "yarn build-clean && yarn build",
     "publish": "yarn build-clean && yarn build && lerna publish"
   },
   "dependencies": {

packages/cli/src/core/findPlugins.js

@@ -5572,15 +5572,10 @@ [email protected]:
   version "0.0.8"
   resolved "http://registry.npmjs.org/minimist/-/minimist-0.0.8.tgz#857fcabfc3397d2625b8228262e86aa7a011b05d"
 
-minimist@^1.1.1, minimist@^1.1.3:
+minimist@^1.1.1, minimist@^1.1.3, minimist@^1.2.0:
   version "1.2.0"
   resolved "http://registry.npmjs.org/minimist/-/minimist-1.2.0.tgz#a35008b20f41383eec1fb914f4cd5df79a264284"
 
-minimist@^1.2.0:
-  version "1.2.0"
-  resolved "https://registry.yarnpkg.com/minimist/-/minimist-1.2.0.tgz#a35008b20f41383eec1fb914f4cd5df79a264284"
-  integrity sha1-o1AIsg9BOD7sH7kU9M1d95omQoQ=
-
 minimist@~0.0.1:
   version "0.0.10"
   resolved "http://registry.npmjs.org/minimist/-/minimist-0.0.10.tgz#de3f98543dbf96082be48ad1a0c7cda836301dcf"