Generalise package main resolving issue

Resolves jestjs#5967

integration-tests/resolve-node-module/__mocks__/mock-module-alt/index.js

@@ -0,0 +1,8 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc. All rights reserved.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+module.exports = 'test';

integration-tests/resolve-node-module/__mocks__/mock-module-alt/package.json

@@ -0,0 +1,4 @@
+{
+  "name": "mock-module",
+  "main": "./"
+}

integration-tests/resolve-node-module/__tests__/resolve-node-module.test.js

@@ -15,7 +15,12 @@ it('should resolve entry as index.js when package main is "."', () => {
   expect(mockModule).toEqual('test');
 });
 
-it('should resolve entry as index with other configured module file extention when package main is "."', () => {
+it('should resolve entry as index.js when package main is "./"', () => {
+  const mockModule = require('mock-module-alt');
+  expect(mockModule).toEqual('test');
+});
+
+it('should resolve entry as index with other configured module file extension when package main is "."', () => {
   const mockJsxModule = require('mock-jsx-module');
   expect(mockJsxModule).toEqual('test jsx');
 });

packages/jest-resolve/src/default_resolver.js

@@ -129,7 +129,7 @@ function resolveSync(target: Path, options: ResolverOptions): Path {
       pkgmain = JSON.parse(body).main;
     } catch (e) {}
 
-    if (pkgmain && pkgmain !== '.') {
+    if (pkgmain && !isCurrentDirectory(pkgmain)) {
       const resolveTarget = path.resolve(name, pkgmain);
       const result = tryResolve(resolveTarget);
       if (result) {
@@ -175,3 +175,7 @@ function isDirectory(dir: Path): boolean {
 
   return result;
 }
+
+function isCurrentDirectory(testPath: Path): boolean {
+  return path.resolve('.') === path.resolve(testPath);
+}