fix: move the plugin-install hook handler out of the init folder

The init folder is for init hook handlers.

package.json

@@ -88,7 +88,7 @@
         "./src/hooks/init/plugin-verification"
       ],
       "plugins:preinstall": [
-        "./src/hooks/init/plugin-install"
+        "./src/hooks/plugin-install"
       ]
     },
     "macos": {

test/hooks/plugin-install.test.js

@@ -0,0 +1,61 @@
+const sinon = require('sinon');
+const inquirer = require('inquirer');
+const pluginFunc = require('../../src/hooks/plugin-install');
+const { expect, test } = require('@twilio/cli-test');
+
+const getNonTwilioPlugin = () => ({
+  plugin: {
+    name: '@twilio-labs-h4x0r/plugin-serverless'
+  }
+});
+
+const getTwilioPlugin = () => ({
+  plugin: {
+    name: '@twilio/debugger'
+  }
+});
+
+const getTwilioLabsPlugin = () => ({
+  plugin: {
+    name: '@twilio-labs/debugger'
+  }
+});
+
+const getUndefinedPlugin = () => ({
+  plugin: {
+    name: undefined
+  }
+});
+
+describe('hooks', () => {
+  describe('plugin-install', () => {
+    before(() => {
+      inquirer._prompt = inquirer.prompt;
+      inquirer.prompt = sinon.stub().resolves({ continue: false });
+    });
+
+    after(() => {
+      inquirer.prompt = inquirer._prompt;
+      delete inquirer._prompt;
+    });
+
+    test.stderr().it('warning when non Twilio plugin is installed', async ctx => {
+      ctx.exit = sinon.stub().resolves(1);
+      await pluginFunc.call(ctx, getNonTwilioPlugin());
+      expect(ctx.stderr).to.contain('WARNING');
+    });
+
+    test.stderr().it('warning when plugin is undefined', async ctx => {
+      ctx.exit = sinon.stub().resolves(1);
+      await pluginFunc.call(ctx, getUndefinedPlugin());
+      expect(ctx.stderr).to.contain('WARNING');
+    });
+
+    test.stderr().it('outputs nothing when Twilio plugin is installed', async ctx => {
+      await pluginFunc.call(ctx, getTwilioPlugin());
+      expect(ctx.stderr).to.be.empty;
+      await pluginFunc.call(ctx, getTwilioLabsPlugin());
+      expect(ctx.stderr).to.be.empty;
+    });
+  });
+});