refactor(2.0): Add ExtraDownloadsRepository

[tienvx]

This is the first PR that add code to use Composer's terms. This one use the term Repository.

• Currently, when a package require (n) extra files, it need (n) tracking files, e.g.
  • path/to/files/text.txt need path/to/files/.composer-downloads/text-9a09b4dfda82e3e665e31092d1c3ec8d.json
  • path/to/files/executable.phar need path/to/files/.composer-downloads/executable-d0ea4c3608f59a037ffbb52f6cf742f5.json
  • path/to/files/extracted-zip/* need path/to/files/.composer-downloads/extracted-zip-78805a221a988e79ef3f42d7c5bfd418.json
• This PR use the same approach with Composer: create vendor/composer/installed-extra-downloads.json, similar to, but much smaller than vendor/composer/installed.json (only contains tracking checksum, not able to re-construct package objects from data)
• Implementing InstalledRepositoryInterface interface is required, but we only use half of its methods. Other methods just return null, 0 (zero), [] (empty array), or "a string". Hopefully it doesn't violate L (Liskov substitution principle) in SOLID
• User need to deleted downloaded files manually before new files can be downloaded, because:
  • Old tracking files are not used, new tracking file is not created
  • Overwriting file protection: can't overwrite files that are not tracked. This info message will be displayed: <info>Extra file <comment>extra-file</comment> has been locally overriden in <comment>path/to/extra-file</comment>. To reset it, delete and reinstall.</info>