Require puppet/module_tool to access Puppet::ModuleTool on Puppet 3.5.0

In Puppet 3.5.0, it seems Puppet::ModuleTool is no longer accessible when
requiring 'puppet' alone, so specify the top level file that brings in
this namespace.  Otherwise you'll get:

lib/librarian/puppet/source/local.rb:78:in `evaluate_modulefile': uninitialized constant Puppet::ModuleTool (NameError)
  from lib/librarian/puppet/source/local.rb:74:in `module_version'
  from lib/librarian/puppet/source/local.rb:42:in `fetch_version'