Plugin Manager in Core: Part 4 (installation profiles)

This is one of four issues to get Plugin Manager in core. The Plugin Manager module in contributions can be found at http://drupal.org/project/plugin_manager. It would be a huge boost to usability if we didn't have to mess around with our file system before we do updates or new module/theme installations. The reason I've separated this into four issues is because otherwise, this will quite quickly turn into even more of a monster patch. I will be updating my patches every Tuesday and Saturday, so getting reviews between each of these regular updates would be awesome :). The four issues I've created are outlined as below:

1. #395472: Plugin Manager in Core: Part 1 (backend) →
2. #395474: Plugin Manager in Core: Part 2 (integration with update status) →
3. #395478: Plugin Manager in Core: Part 3 (integration with installation system) →
4. 📌 Plugin Manager in Core: Part 4 (installation profiles) Postponed: needs info

For more details on the other issues, click the above links.

This issue will deal with integration of installation profiles with the new plugin manager system. The current plugin manager module doesn't do this, but this will open up all sorts of possibilities. First of all, rather than provide a limited subset of core installation profiles during initial installation, we could list *all* applicable installation profiles on drupal.org. This way, we greatly boost the visibility of install profiles - the goal of them is supposed to be to make it easier for a Drupal newbie to set up a site of a particular type, but they don't do much good if the vast majority of Drupal newbies are ignorant of their existence. This is certainly at least part of the reason why we haven't seen many install profiles (along with the need for better API functions).

A second possibility (in addition to the first one) would be using plugin manager to fetch any necessary modules (and themes?) from drupal.org, and install them automatically with the profile. This would save profiles from the necessity of either shipping with all the required contributed modules (and updating whenever any of those modules comes out with a new release) or making its users download any contributed modules themselves (tedious and bad user experience). This issue will explore this possibility as well as the first one, though it may later be split off into a fifth issue.