Abandoned Package Warnings

Now that packagist can warn users about packages that are no longer in development, we need to warn people who are using them to look for alternatives or fork the packages before they go missing from github.

This PR adds this in different places:
- abandoned data is now processed into the `CompletePackage` object
- `composer show` will show warning if viewing abandoned package
- `composer install|update` will show warnings for each abandoned package in its local install.

This uses the functionality in composer/packagist#192 and may partially solve #1114.