fix: avoid ES6 export for entry files w/o exports (#351)

No ES6 export is added to _entry_ files containing no CommonJS export.

E.g. this:
```
var dummy = require('./dummy');
var input = 42;
```

Is now transformed into the following containing no ES6 export:
```
import './dummy';
import dummy from 'commonjs-proxy:./dummy';
var input = 42;
```

Author: opichals

src/transform.js

@@ -459,7 +459,7 @@ export function transformCommonjs(
 			}
 		});
 
-		if (!hasDefaultExport) {
+		if (!hasDefaultExport && (names.length || !isEntry)) {
 			wrapperEnd = `\n\nvar ${moduleName} = {\n${names
 				.map(({ name, deconflicted }) => `\t${name}: ${deconflicted}`)
 				.join(',\n')}\n};`;
@@ -488,7 +488,11 @@ export function transformCommonjs(
 		.trim()
 		.prepend(importBlock + wrapperStart)
 		.trim()
-		.append(wrapperEnd + exportBlock);
+		.append(wrapperEnd);
+
+	if (hasDefaultExport || named.length > 0 || shouldWrap || !isEntry) {
+		magicString.append(exportBlock);
+	}
 
 	code = magicString.toString();
 	const map = sourceMap ? magicString.generateMap() : null;

test/form/no-exports-entry/_config.js

@@ -0,0 +1,6 @@
+module.exports = {
+	options: {
+		ignore: [ 'foo' ]
+	},
+	entry: './input.js'
+};

test/form/no-exports-entry/input.js

@@ -0,0 +1,7 @@
+var dummy = require('./dummy');
+
+var foo = function () {
+	return;
+};
+
+var input = 42;

test/form/no-exports-entry/output.js

@@ -0,0 +1,8 @@
+import './dummy';
+import dummy from '_./dummy?commonjs-proxy';
+
+var foo = function () {
+	return;
+};
+
+var input = 42;

test/test.js

@@ -69,27 +69,22 @@ async function executeBundle(bundle, { context, exports } = {}) {
 	return execute(code, context);
 }
 
-const transformContext = {
-	getModuleInfo(id) {
-		return {
-			isEntry: id === 'main.js'
-		};
-	},
-	parse: (input, options) =>
-		acorn.parse(
-			input,
-			Object.assign(
-				{
-					ecmaVersion: 9,
-					sourceType: 'module'
-				},
-				options
-			)
-		)
-};
-
 describe('rollup-plugin-commonjs', () => {
 	describe('form', () => {
+		const transformContext = {
+			parse: (input, options) =>
+				acorn.parse(
+					input,
+					Object.assign(
+						{
+							ecmaVersion: 9,
+							sourceType: 'module'
+						},
+						options
+					)
+				)
+		};
+
 		fs.readdirSync('form').forEach(dir => {
 			let config;
 
@@ -101,8 +96,13 @@ describe('rollup-plugin-commonjs', () => {
 
 			(config.solo ? it.only : it)(dir, () => {
 				const { transform } = commonjs(config.options);
+				const id = `./form/${dir}/input.js`;
 
-				const input = fs.readFileSync(`form/${dir}/input.js`, 'utf-8');
+				transformContext.getModuleInfo = moduleId => ({
+					isEntry: config.entry && moduleId === id
+				});
+
+				const input = fs.readFileSync(id, 'utf-8');
 
 				let outputFile = `form/${dir}/output`;
 				if (fs.existsSync(`${outputFile}.${process.platform}.js`)) {
@@ -112,7 +112,8 @@ describe('rollup-plugin-commonjs', () => {
 				}
 
 				const expected = fs.readFileSync(outputFile, 'utf-8').trim();
-				const transformed = transform.call(transformContext, input, 'input.js');
+
+				const transformed = transform.call(transformContext, input, id);
 				const actual = (transformed ? transformed.code : input).trim().replace(/\0/g, '_');
 				assert.equal(actual, expected);
 			});