feat: add support for Serverless node devDeps exclusion feature

Serverless 0.16.0 introduced a feature which excludes Node.js dev dependencies from .zip files. The feature uses package.json to find which dependencies can be excluded.
serverless/serverless#3737

This commit adds the package.json to the .build folder, so that Serverless can find the package.json file.

Author: adieuadieu

example/package.json

@@ -1,7 +1,9 @@
 {
   "dependencies": {
+    "lodash": "^4.17.4"
+  },
+  "devDependencies": {
     "@types/lodash": "^4.14.63",
-    "lodash": "^4.17.4",
     "serverless-plugin-typescript": "^0.1.2"
   }
 }

src/index.ts

@@ -91,6 +91,11 @@ class ServerlessPlugin {
       fs.symlinkSync(path.resolve('node_modules'), path.resolve(path.join(buildFolder, 'node_modules')))
     }
 
+    // include package.json into build so Serverless can exlcude devDeps during packaging
+    if (!fs.existsSync(path.resolve(path.join(buildFolder, 'package.json')))) {
+      fs.symlinkSync(path.resolve('package.json'), path.resolve(path.join(buildFolder, 'package.json')))
+    }
+
     // include any "extras" from the "include" section
     if (this.serverless.service.package.include && this.serverless.service.package.include.length > 0) {
       const files = await globby(this.serverless.service.package.include)