This updates TFSClient to add formal support for dependency checks.

Unlike most other SCM updates, this one's a bit more complicated due to

the three wrapper classes that can be chosen.
The wrapper classes now all inherit from a base class, which defines the

interface for wrappers. They then override their own

check_dependencies(), performing the checks that used to be done in

TFSClient.
TFSClient has been updated to check each wrapper in order, selecting

one that passes dependency checks, or failing with a hopefully helpful

error listing broad dependencies.
That has the added benefit of now allowing a fallback to TEE when the TF

Helper is installed but Java is not.
The tf_wrapper attribute is now a property, which enforces a

dependency check, emitting a warning if dependencies haven't already

been checked via setup() or has_dependencies(). All methods on

TFSClient go through this already, ensuring this is used.
There weren't any TFS unit tests before. This change adds the first

ones, which are all focused on the variety of dependency checks.

Unit tests pass on Python 3.7-3.11.