Request yarn to be available for launch

This change adds supports to cases where yarn is used in start
commands. A common example is the use of yarn workspaces or
frameworks (like lerna) built on top of yarn workspaces.

A chose yarn command is run in a particular workspace using the
following syntax:
`yarn workspace <workspace_name> <command>`
See https://classic.yarnpkg.com/en/docs/cli/workspace/
This requires the yarn tool to be available at launch time.
See added integration test for an example.

Also, see issue paketo-buildpacks/nodejs#456

.gitignore

@@ -1,2 +1,4 @@
 /.bin
 /.build
+/bin
+/build

constants.go

@@ -3,4 +3,5 @@ package yarnstart
 const (
 	Node        = "node"
 	NodeModules = "node_modules"
+	Yarn        = "yarn"
 )

detect.go

@@ -37,6 +37,12 @@ func Detect(projectPathParser PathParser) packit.DetectFunc {
 							"launch": true,
 						},
 					},
+					{
+						Name: Yarn,
+						Metadata: map[string]interface{}{
+							"launch": true,
+						},
+					},
 					{
 						Name: NodeModules,
 						Metadata: map[string]interface{}{

detect_test.go

@@ -57,6 +57,12 @@ func testDetect(t *testing.T, context spec.G, it spec.S) {
 							"launch": true,
 						},
 					},
+					{
+						Name: "yarn",
+						Metadata: map[string]interface{}{
+							"launch": true,
+						},
+					},
 					{
 						Name: "node_modules",
 						Metadata: map[string]interface{}{

integration/init_test.go

@@ -85,5 +85,6 @@ func TestIntegration(t *testing.T) {
 	suite("Default", testDefault)
 	suite("GracefulShutdown", testGracefulShutdown)
 	suite("ProjectPath", testProjectPath)
+	suite("Workspaces", testWorkspaces)
 	suite.Run(t)
 }

integration/testdata/workspaces_app/package.json

@@ -0,0 +1,10 @@
+{
+  "main": "packages/sample-app/index.js",
+  "scripts": {
+    "start": "yarn workspace @sample/sample-app start"
+  },
+  "private": true,
+  "workspaces": [
+    "packages/*"
+  ]
+}

integration/testdata/workspaces_app/packages/sample-app/index.js

@@ -0,0 +1,13 @@
+const express = require('express');
+const app = express();
+const config = require('@sample/sample-config');
+
+app.get('/', (req, res) => {
+    res.send({
+        config: config(),
+    });
+});
+
+const port = process.env.PORT || 8080;
+
+app.listen(port, () => console.log(`Sample app listening on port ${ port }!`));

integration/testdata/workspaces_app/packages/sample-app/package.json

@@ -0,0 +1,12 @@
+{
+  "name": "@sample/sample-app",
+  "version": "1.0.0",
+  "main": "index.js",
+  "scripts": {
+    "start": "node index.js"
+  },
+  "dependencies": {
+    "@sample/sample-config": "^1.0.0",
+    "express": "^4.16.3"
+  }
+}

integration/testdata/workspaces_app/packages/sample-config/index.js

@@ -0,0 +1,8 @@
+const config = () => {
+    return {
+        prop1: 'Package A value 1',
+        prop2: 'Package A value 2',
+    };
+};
+
+module.exports = config;

integration/testdata/workspaces_app/packages/sample-config/package.json

@@ -0,0 +1,5 @@
+{
+  "name": "@sample/sample-config",
+  "version": "1.0.0",
+  "main": "index.js"
+}