Support inline aliases for downloading files

[deviantintegral]

We're patching core in a separate repository instead of using composer patches. However, this breaks downloading the scaffold files, as the package version is used directly instead of any aliases. That means that files are attempted to be downloaded from drupal.org with our custom branch or tag, instead of what best corresponds to the fork.

This PR respects the version as version syntax in composer.json for determining what to download.

[deviantintegral]

It looks like master is failing tests: https://travis-ci.org/drupal-composer/drupal-scaffold/builds