Fixes #6559: Net::DHCP classloading error

There are always weird errors like this when requiring and autoloading is combined. I think for this to work correctly there is couple of options:
- to have lib dir without autoloading which can be safely required. 
- loading it by Constant as usual autoloading, making sure there are no name/space conflicts (FQN for constants may have to used)
- there is also `require_dependency` or something like that provided by active support autoloading for these situations, but I do not fully understand its behavior.

I think 1 and 2 are the safest options without any AS magic.