[itamarst]

Fixes #277

Previously prefix was used as the controlling key to identify LibControllers, but prefix is not guaranteed to be unique. Thus the switch to filepath.

While I was at it, I deleted some code that did unnecessary work of setting unused private attributes, possibly remnants of some old refactoring.

This is the companion to #275 so joblib can more reliably say "OpenBLAS+pthreads, if used, will be either 1 or cpu_count() cores and that's it". Given this change, joblib would do something like this:

def set_num_threads(num_threads):
    tp_controller = get_cached_controller()
    limits = []
    for lib_controller in tp_controller.lib_controllers:
        # filepath for threadpoolctl that has this PR, prefix for backwards compat with older versions
        limit = {"filepath": lib_controller.filepath, "prefix": lib_controller.prefix}
        if isinstance(lib_controller, OpenBLASController) and lib_controller.threading_layer != "openmp":
            cpus = cpu_count(process_wide=True)
            if num_threads >= 0.75 * cpus:
                limit["num_threads"] = 1
            else:
                limit["num_threads"] = cpus
        else:
            limit["num_threads"] = num_threads
        limits.append(limit)
    tp_controller.limit(limits=limits)