Add compatibility with puppet-stdlib ensure_packages function

Logically, code is the same as before. 

But compatibility with puppet-stdlib ensure_packages/ensure_resource is on board.

Small snippet to explain commit:

```puppet
node foo {
  include rvm
  include another_module
}
```

```puppet
class another_module 
{
  ensure_packages(['libxml2','bar']) # There is no redeclaration error with this commit 
}
```