Support multiple APT preferences snippets for the same package name pattern.

This is implemented by adding a "package" parameter to apt::preferences_snippet,
so that define names can be kept unique while the package names are
not necessarily.

Closes: Redmine#3468.

README

@@ -314,6 +314,13 @@ Example:
       priority => 999;
   }
 
+  apt::preferences_snippet{
+    'unstable_fallback':
+      package => '*',
+      release => 'unstable',
+      priority => 1;
+  }
+
 apt::preseeded_package
 ----------------------
 

manifests/preferences_snippet.pp

@@ -1,4 +1,5 @@
 define apt::preferences_snippet(
+  $package = $name,
   $ensure = 'present',
   $source = '',
   $release,

templates/preferences_snippet.erb

@@ -1,4 +1,4 @@
-Package: <%= name %>
+Package: <%= package %>
 Pin: release a=<%= release %>
 Pin-Priority: <%= priority %>