This changes the 'name' strategy to always align the local configured
workspace name and the remote Terraform Cloud workspace, rather than the
implicit use of the 'default' unnamed workspace being used instead.

What this essentially means is that the Cloud integration does not fully
support workspaces when configured for a single TFC workspace (as was
the case with the 'remote' backend), but *does* use the
backend.Workspaces() interface to allow for normal local behaviors like
terraform.workspace to resolve to the correct name. It does this by
always setting the local workspace name when the 'name' strategy is
used, as a part of initialization.

Part of the diff here is exporting all the previously unexported types
for mapping workspaces. The command package (and init in particular)
needs to be able to handle setting the local workspace in this
particular scenario.