We'll soon be in a position where one of our bundled SCMTools will need

its name changed. Doing so would result in Tool DB entry conflicts and

registry lookup issues, impacting the API and repositories.
To prepare for this, a new attribute is being introduced to provide a

stable name for lookup purposes: SCMTool.lookup_name.
This defaults to SCMTool.name (by way of an overridable

classproperty). Subclasses can override this to specify an explicit

name when they're going to change their name property.
lookup_name is used for registration, lookup, conflict resolution, and

Tool object creation. This ensures that, behind the scenes, nothing

changes for any tools. We simply need to make sure we set lookup_name

on any SCMTool when we're ready to change name.

Unit tests pass.
Tested this with the upcoming Plastic work, ensuring that the tool and

all registry-related operations worked when changing name and setting

lookup_name.