All of our SCMClients need to check for compatible tools to wrap, but we've never really had a formal way of doing these checks. We eventually sort of standardized on doing them in `get_local_path()`, but this both meant that tools couldn't be checked up-front, and that tools would re-check every time we wanted to check the path. We dealt with this through caching of information, but it wasn't great. This approach also had a problem where `get_local_path()` was doing the GNU diff checks. With Apple Diff support coming, we needed a better way of checking the diff tool, outside of the SCMClient. That led to a rethink on dependency checks. SCMClients should now implement `check_dependencies()` to do their tool existence/version checks. This method must raise a `ClientDependencyError` exception if all mandatory dependencies fail. It may store any data on the instance or on a class to indicate, for example, which executable name or tool version it's working with. It also must *not* check for GNU diff, as that will be a check performed outside of the SCMClients. When instantiating a tool, it's now required to run either `scm_client.setup()` or `scm_client.has_dependencies()`. `BaseSCMClient.setup()` will check for dependencies the first time it's called and will store whether the tool has dependencies. If dependency checks fail, it will re-raise the exception. Callers should explicitly call `setup()` if they are interested in the details of the missing dependencies. This method must not be overridden. If SCMClients need additional setup hooks, that's something we'd need to provide separately for `setup()` to call. `BaseSCMClient.has_dependencies()` will call `setup()` if needed. It then returns a boolean indicating if dependencies are present. This is a good alternative if the caller just needs to check the usability of a tool before doing anything else with it. As of RBTools 4.0, clients can be used without calling `setup()`, but a deprecation warning will be issued. This will help transition to the new behavior. This takes place in `has_dependencies()` with the default parameter of `expect_checked=True`. In RBTools 5, this will raise an exception instead. Upcoming changes will begin to transition over to this for specifying dependencies, for simplifying `get_local_path()`, and for calling `setup()`.