(PDOC-3) Use require instead of require_relative

Prior to this commit, the puppet strings project was using require_relative
to include other classes and modules. Due to some ambiguity and the fact
that the strings project shares some naming conventions with YARD, the
use of require_relative sometimes made it unclear what was being
included and where it was coming from. Now require is being used and although
this requires the full path, it makes it very clear where things are coming
from.