fix: use path.posix for better cross OS compat#4

## Summary The bundler currently crashes on my Windows machine due to OS differences resulting in the generated output to not work as intended: ``` C:\Users\yangshun\developer\tmp\byo-bundler\output\bundle.js:32 modules[moduleName](exports, require); ^ TypeError: modules[moduleName] is not a function at require (C:\Users\yangshun\developer\tmp\byo-bundler\output\bundle.js:32:24) at Object.fixture/index.js (C:\Users\yangshun\developer\tmp\byo-bundler\output\bundle.js:1:85) ... ``` See the "Before" section in the Test Plan to see why. The fix is to use `path.posix` for consistent path handling. Alternatively, in Node 15, you can just import `const path = require('path/posix')` but since Node 15 is still quite new I think it's better to use the older API. ## Test Plan ### Before ```js const modules = {"C:\Users\yangshun\developer\tmp\byo-bundler\fixture\index.js": function(exports, require) { const _imported = require("C:\\Users\\yangshun\\developer\\tmp\\byo-bundler\\fixture\\square.js"); const _imported2 = require("C:\\Users\\yangshun\\developer\\tmp\\byo-bundler\\fixture\\circle.js"); exports.PI = 3.141; console.log('Area of square: ', _imported["default"](5)); console.log('Area of circle', _imported2["default"](5)); },"C:\Users\yangshun\developer\tmp\byo-bundler\fixture\square.js": function(exports, require) { function area(side) { return side * side; } exports.default = area; },"C:\Users\yangshun\developer\tmp\byo-bundler\fixture\circle.js": function(exports, require) { const _imported = require("C:\\Users\\yangshun\\developer\\tmp\\byo-bundler\\fixture\\index.js"); function area(radius) { return _imported["PI"] * radius * radius; } exports.default = area; },}; const entry = "C:\Users\yangshun\developer\tmp\byo-bundler\fixture\index.js"; function webpackStart({ modules, entry }) { const moduleCache = {}; const require = moduleName => { // if in cache, return the cached version if (moduleCache[moduleName]) { return moduleCache[moduleName]; } const exports = {}; // this will prevent infinite "require" loop // from circular dependencies moduleCache[moduleName] = exports; // "require"-ing the module, // exported stuff will assigned to "exports" modules[moduleName](exports, require); return moduleCache[moduleName]; }; // start the program require(entry); } webpackStart({ modules, entry }); ``` ### After ```js const modules = {"fixture/index.js": function(exports, require) { const _imported = require("fixture/square.js"); const _imported2 = require("fixture/circle.js"); exports.PI = 3.141; console.log('Area of square: ', _imported["default"](5)); console.log('Area of circle', _imported2["default"](5)); },"fixture/square.js": function(exports, require) { function area(side) { return side * side; } exports.default = area; },"fixture/circle.js": function(exports, require) { const _imported = require("fixture/index.js"); function area(radius) { return _imported["PI"] * radius * radius; } exports.default = area; },}; const entry = "fixture/index.js"; function webpackStart({ modules, entry }) { const moduleCache = {}; const require = moduleName => { // if in cache, return the cached version if (moduleCache[moduleName]) { return moduleCache[moduleName]; } const exports = {}; // this will prevent infinite "require" loop // from circular dependencies moduleCache[moduleName] = exports; // "require"-ing the module, // exported stuff will assigned to "exports" modules[moduleName](exports, require); return moduleCache[moduleName]; }; // start the program require(entry); } webpackStart({ modules, entry }); ```