better extension lookup

[jonathanong]

should go through all the fields in the order defined and look for:

• exact match file.path === path
• extensionless match file.path.replace(/\.\w+$/, '') === path
• folder-index match file.path.replace(/\/index\.\w+$/, '') === path

making it extension-agnostic unlike node's ['', '.js', '.json', '/index.js'] lookup, which this builder currently does

[timaschew]

if you have this directory structure:

foo.js
foo/
  index.js

require("./foo") would match the file and not the directory is that desired?