Sites symlinks

This PR has more than a few small tweaks in it, but here is the summary of each commit's outcomes:

- Feature: Use `sites-enabled` and `sites-available` correctly on Debian (and fix `mods-enabled` loading)
- Bugfix: Load `vhost_alias` by default if `apache::vhost::virtual_docroot` is passed
- Bugfix: Correctly manage `php5.conf` for `apache::mod::php` on Debian
- Bugfix: Correctly ensure/purge config, vhost, & mod directories before/after package installation. (Avoiding `<||>` because of virtual resource realization.)

The bugfixes were discovered during the introduction of this feature.