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/dependencies/umd_dependency_host_spec.ts

@@ -115,6 +115,16 @@ runInEachFileSystem(() => {
         expect(missing.size).toBe(0);
         expect(deepImports.size).toBe(0);
       });
+
+      it('should resolve path to the file if it has no extension', () => {
+        const {dependencies, missing, deepImports} =
+            host.findDependencies(_('/external/imports/index'));
+        expect(dependencies.size).toBe(2);
+        expect(missing.size).toBe(0);
+        expect(deepImports.size).toBe(0);
+        expect(dependencies.has(_('/node_modules/lib_1'))).toBe(true);
+        expect(dependencies.has(_('/node_modules/lib_1/sub_1'))).toBe(true);
+      });
     });
 
     function setupMockFileSystem(): void {