[Fix] make /node_modules/ check work on windows

Author: bgotink

lib/resolve-exports.js

@@ -58,7 +58,7 @@ if (typeof String.prototype.startsWith === 'function') {
 function resolveExportsTarget(packagePath, parent, key, target, subpath, env) {
     if (typeof target === 'string') {
         var resolvedTarget = path.resolve(packagePath, target);
-        if (!(/^\.\//).test(target) || resolvedTarget.indexOf('/node_modules/', packagePath.length - 1) !== -1 || !startsWith(resolvedTarget, packagePath)) {
+        if (!(/^\.\//).test(target) || resolvedTarget.indexOf(path.sep + 'node_modules' + path.sep, packagePath.length - 1) !== -1 || !startsWith(resolvedTarget, packagePath)) {
             throw makeError('ERR_INVALID_PACKAGE_TARGET', 'Invalid "exports" target ' + JSON.stringify(target)
                     + ' defined for ' + key + ' in ' + packagePath + path.sep + 'package.json imported from ' + parent + '.');
         }