Add missing .npmignore files

Summary:
I noticed some of our packages publish `src.real` to NPM, which is an intermediate build artefact that shouldn't published. It turns out this is because a few of them are missing the standard `.npmignore`, and so also publishing test code and lock files.

This adds the missing `.npmignore` files, and adds a check to our `subpackages-test.js` for existence and minimal content.

Reviewed By: motiz88

Differential Revision: D36592818

fbshipit-source-id: adac01c5c9d632e3388290a46c3cbc3308b4e239

Author: robhogan

packages/metro-cache-key/.npmignore

@@ -0,0 +1,5 @@
+**/__mocks__/**
+**/__tests__/**
+build
+src.real
+yarn.lock

packages/metro-symbolicate/.npmignore

@@ -0,0 +1,5 @@
+**/__mocks__/**
+**/__tests__/**
+build
+src.real
+yarn.lock

packages/metro-transform-worker/.npmignore

@@ -0,0 +1,5 @@
+**/__mocks__/**
+**/__tests__/**
+build
+src.real
+yarn.lock

scripts/__tests__/subpackages-test.js

@@ -81,3 +81,20 @@ it('forces all packages to have a src/ folder', () => {
     );
   });
 });
+
+it('forces all packages to have an .npmignore with expected entries', () => {
+  checkAssertionInPackages(getPackages(), packagePath => {
+    const npmIgnorePath = path.join(packagePath, '.npmignore');
+    expect(fs.existsSync(npmIgnorePath)).toBe(true);
+    const lines = fs.readFileSync(npmIgnorePath, 'utf-8').split('\n');
+    expect(lines).toEqual(
+      expect.arrayContaining([
+        '**/__mocks__/**',
+        '**/__tests__/**',
+        'build',
+        'src.real',
+        'yarn.lock',
+      ]),
+    );
+  });
+});