Rename "BuildStepImplementation" to "HarbormasterBuildStepImplementation"

Summary: Ref T1049. D8588 already required custom code to change what it extends, so this is as good a time as we're going to get to move to more standard class name.

Test Plan: arc liberate; arc lint

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D8601