[CHEF-2584] Fix portage install error when same name packages exist

http://tickets.opscode.com/browse/CHEF-2584

fixed following:

for example, "dev-db/mysql" and "virtual/mysql" exist

```ruby
package "dev-db/mysql" do
  action :install
end

# => Chef::Exceptions::Package, "Multiple packages found for ..."
```
even if only package_without_category match, package is added to availables,
and so multiple packages found.