fix(ivy): ngcc - resolve main property paths correctly (#31509)

When determining if a `main` path points to a UMD or CommonJS
format, the contents of the file need to be loaded and parsed.

Previously, it was assumed that the path referred to the exact filename,
but did not account for normal module resolution semantics, where the
path may be missing an extension or refer to a directory containing an
`index.js` file.

// FW-1444

PR Close #31509

packages/compiler-cli/ngcc/test/packages/entry_point_spec.ts

@@ -10,7 +10,7 @@ import {AbsoluteFsPath, FileSystem, absoluteFrom, getFileSystem} from '../../../
 import {runInEachFileSystem} from '../../../src/ngtsc/file_system/testing';
 import {loadTestFiles} from '../../../test/helpers';
 import {NgccConfiguration} from '../../src/packages/configuration';
-import {SUPPORTED_FORMAT_PROPERTIES, getEntryPointInfo} from '../../src/packages/entry_point';
+import {EntryPoint, SUPPORTED_FORMAT_PROPERTIES, getEntryPointFormat, getEntryPointInfo} from '../../src/packages/entry_point';
 import {MockLogger} from '../helpers/mock_logger';
 
 runInEachFileSystem(() => {
@@ -340,6 +340,92 @@ runInEachFileSystem(() => {
     });
   });
 
+  describe('getEntryPointFormat', () => {
+    let SOME_PACKAGE: AbsoluteFsPath;
+    let _: typeof absoluteFrom;
+    let fs: FileSystem;
+    let entryPoint: EntryPoint;
+
+    beforeEach(() => {
+      _ = absoluteFrom;
+      SOME_PACKAGE = _('/project/node_modules/some_package');
+      fs = getFileSystem();
+      loadTestFiles([{
+        name: _('/project/node_modules/some_package/valid_entry_point/package.json'),
+        contents: createPackageJson('valid_entry_point')
+      }]);
+      const config = new NgccConfiguration(fs, _('/project'));
+      entryPoint = getEntryPointInfo(
+          fs, config, new MockLogger(), SOME_PACKAGE,
+          _('/project/node_modules/some_package/valid_entry_point')) !;
+    });
+
+    it('should return `esm2015` format for `fesm2015` property',
+       () => { expect(getEntryPointFormat(fs, entryPoint, 'fesm2015')).toBe('esm2015'); });
+
+    it('should return `esm5` format for `fesm5` property',
+       () => { expect(getEntryPointFormat(fs, entryPoint, 'fesm5')).toBe('esm5'); });
+
+    it('should return `esm2015` format for `es2015` property',
+       () => { expect(getEntryPointFormat(fs, entryPoint, 'es2015')).toBe('esm2015'); });
+
+    it('should return `esm2015` format for `esm2015` property',
+       () => { expect(getEntryPointFormat(fs, entryPoint, 'esm2015')).toBe('esm2015'); });
+
+    it('should return `esm5` format for `esm5` property',
+       () => { expect(getEntryPointFormat(fs, entryPoint, 'esm5')).toBe('esm5'); });
+
+    it('should return `esm5` format for `module` property',
+       () => { expect(getEntryPointFormat(fs, entryPoint, 'module')).toBe('esm5'); });
+
+    it('should return `umd` for `main` if the file contains a UMD wrapper function', () => {
+      loadTestFiles([{
+        name: _(
+            '/project/node_modules/some_package/valid_entry_point/bundles/valid_entry_point/index.js'),
+        contents: `
+        (function (global, factory) {
+          typeof exports === 'object' && typeof module !== 'undefined' ? factory(exports, require('@angular/core')) :
+          typeof define === 'function' && define.amd ? define('@angular/common', ['exports', '@angular/core'], factory) :
+          (global = global || self, factory((global.ng = global.ng || {}, global.ng.common = {}), global.ng.core));
+        }(this, function (exports, core) { 'use strict'; }));
+      `
+      }]);
+      expect(getEntryPointFormat(fs, entryPoint, 'main')).toBe('umd');
+    });
+
+    it('should return `commonjs` for `main` if the file does not contain a UMD wrapper function', () => {
+      loadTestFiles([{
+        name: _(
+            '/project/node_modules/some_package/valid_entry_point/bundles/valid_entry_point/index.js'),
+        contents: `
+          const core = require('@angular/core);
+          module.exports = {};
+        `
+      }]);
+      expect(getEntryPointFormat(fs, entryPoint, 'main')).toBe('commonjs');
+    });
+
+    it('should resolve the format path with suitable postfixes', () => {
+      loadTestFiles([{
+        name: _(
+            '/project/node_modules/some_package/valid_entry_point/bundles/valid_entry_point/index.js'),
+        contents: `
+        (function (global, factory) {
+          typeof exports === 'object' && typeof module !== 'undefined' ? factory(exports, require('@angular/core')) :
+          typeof define === 'function' && define.amd ? define('@angular/common', ['exports', '@angular/core'], factory) :
+          (global = global || self, factory((global.ng = global.ng || {}, global.ng.common = {}), global.ng.core));
+        }(this, function (exports, core) { 'use strict'; }));
+      `
+      }]);
+
+      entryPoint.packageJson.main = './bundles/valid_entry_point/index';
+      expect(getEntryPointFormat(fs, entryPoint, 'main')).toBe('umd');
+
+      entryPoint.packageJson.main = './bundles/valid_entry_point';
+      expect(getEntryPointFormat(fs, entryPoint, 'main')).toBe('umd');
+    });
+  });
+
   function createPackageJson(
       packageName: string, {excludes}: {excludes?: string[]} = {},
       typingsProp: string = 'typings'): string {
@@ -351,7 +437,7 @@ runInEachFileSystem(() => {
       es2015: `./es2015/${packageName}.js`,
       fesm5: `./fesm5/${packageName}.js`,
       esm5: `./esm5/${packageName}.js`,
-      main: `./bundles/${packageName}.umd.js`,
+      main: `./bundles/${packageName}/index.js`,
       module: './index.js',
     };
     if (excludes) {