[DX] Provide a utility for retrieving a single project's info (and settings?)

We currently have only list_themes() which not only is poorly named (backdrop_list_themes maybe?) but also retrieves theme data for all themes. Most of the time, we only need the data for a single theme, resulting in yuck in our code that looks like this....

    global $theme;
    $theme_data = list_themes();
    $theme_object = $theme_data[$theme];

It would be good if we could add backdrop_get_theme_info($themename) that would return the contents of that single info file.

As an added bonus, it could load any files located in the contrib directory of that theme, or in the active directory of the site.