We define SCMTool.scmtool_id as a helper property that computes an ID

if not already set, based on an Entrypoint. This is here because we used

to use entrypoint names as the IDs, and while transitioning away from

that, we needed to maintain compatibility.
We still need this compatibility shim (whether here or in registration,

but at the moment it's here). Power Pack still fails to set an SCMTool

ID for TFS and TFS-Git, so this is still relevant.
Because we assign this to a property, type checkers think the proper

type is that of the helper. This change explicitly marks it as a str.

Unit tests pass.