Add has_module function

[alexjfisher]

The idea is we can use this to do things like...

if extlib::has_module('camptocamp/systemd') {
  include systemd
  systemd::unit_file { 'some.service':
    ...
  }
} else 
{
  # Do something else with execs
}

[eputnam]

@alexjfisher so pumped to see REFERENCE.md! With these examples, you can either escape the hashes (to get rid of the h1 formatting) or use an @example tag to get noformat examples.

[alexjfisher]

Yep, the other functions need revisiting to fix up their output. I was trying not to shave too many yaks this week! :)

I did notice one thing though. Should I have expected my @example to appear in REFERENCE.md?