Support the ability to separate the `forms` list into pre-database and pre-modules groups

The core install system can be broadly divided into two "phases": pre-database, and base-installed.

The "pre-database" phase is the early installer: at this point, no database has been configured, no tables have been set up, and there's no way to persist data at all except to put it into $install_state['parameters'], which is transferred to the URL query string. The recipe kit's RecipesForm and SiteNameForm are handled in this phase.

Once database settings are inputted, the base system is installed (which is really just the System module), and a couple of very basic tables are created. Drupal is then fully bootstrapped. This is a very quick operation, and from this point on, you do have a persistent storage layer and full Drupal API, even though you don't really have any useful modules yet. Once we're in this phase, the username and password for the admin account are set, the remaining modules are installed, and the chosen recipes are applied.

Right now, we only have the ability to inject forms into the pre-database phase. We should also support injecting forms into the base-installed phase, before the recipes are applied.

In the generated info files, this will look like:

forms:
  pre_database:
    - 'Some\Class'
    - 'Some\Other\Class'
  base_install:
    - 'Yet\Another\Class'