(PA-4914) Use relative_require

[joshcooper]

Require will scan each entry in the $LOAD_PATH, so avoid that.

The only remaining requires are for ruby standard gems:

$ git grep -hE "require ['\"][a-z]+['\"]" lib | sort | uniq
require 'forwardable'
require 'optparse'
require 'pathname'
require 'set'
require 'stringio'
require 'uri'