fix: resolve node_modules

[tanhauhau]

• fix resolving scoped modules, eg: @scope/package
  • instead of using / to determine the case of deep imports, use path.extname instead (not sure it's the best way) to know it is importing a package or a file
• fix relative imports within node_modules/.
  • eg: some_package/src/index.js imports './foo.js',
  • because ./foo.js is a relative import, it wasnt rewritten.
  • and if we return some_package/src/index.js when browser requested for '/@modules/some_package', browser will get './foo.js' relative to it, which is '/@modules/foo.js' which is not found.
  • to fix, 302 redirect '/@modules/some_package' to '/@modules/some_package/src/index.js' when resolving, this will help browser to resolve relative files within node_modules

[yyx990803]

Cool! This is neat, it also gets rid of the need for the source map part, which is essentially dealing with relative imports inside modules.