Ensure YardocTask requires yard itself

[jasonkarns]

Description

The yard task is not self contained at the moment and does not run without yard itself first being loaded.

It's preferable for rake tasks to be self-contained (the YardocTask should require its dependencies so users may require just the tasklib itself).

Fixes #1597

Completed Tasks

• I have read the Contributing Guide.
• The pull request is complete (implemented / written).
• Git commits have been cleaned up (squash WIP / revert commits).
• I wrote tests and ran bundle exec rake locally (if code is attached to PR).
  • rake spec passes locally. No tests added yet. Wanting feedback first before spending time deciphering test suite.

[lsegal]

Based on #1597, the intended API for loading the YardocTask is to require 'yard' so as to not introduce an implicit potential breaking change dependency on the file system structure of the YARD codebase. As such, this change should generally not be necessary. The majority of YARD is loaded lazily (except config loading, which is roughly a no-op if a config is not present), so unless there is specific data that shows a performance issue with this approach, I think it's best to stick with the safer API.