Use drush alias groups as namespaces

This patch enhances alias group files, group.aliases.drushrc.php, such that every alias defined inside is added to a simulated namespace group. For example, if mysite.aliases.drushrc.php defines the aliases 'live', 'stage' and 'dev', then you will now be able to address these aliases as @mysite.live, @mysite.stage and @mysite.dev. @live, @stage and @dev may also still be used, provided that there is no conflict with other aliases of the same name (in which case the behavior remains undefined).

This change is important because it is common for users who are new to alias groups to assume that they work this way. The current behavior is surprising; this patch brings the implemented behavior more in line with the expected behavior. There is practically no impact to existing users, who may continue to use their group files as they have been.