[hal] Use MrcLib to talk to DS (#8858)

Using MrcLib on the robot is going to be the plan for the future, to
make things easier.

MrcLib is how sim is supported going forward. The desktop version of
mrclib can act as a robot server.

This is set up where the mrclib interface is in shared code. On robot,
that is the only backend used. On desktop, a default sim backend is
used. However, the sim plugin can switch that to the real robot backend,
so the robot code will exactly look like a real robot.

simulation/halsim_gui/BUILD.bazel

@@ -155,6 +155,7 @@ package_default_cc_project(
 PKG_CONFIG_DEPS = [
     "//datalog:native/datalog/robotpy-native-datalog.pc",
     "//datalog:robotpy-wpilog.generated_pkgcfg_files",
+    "//hal:native/wpihal/robotpy-native-mrclib.pc",
     "//hal:native/wpihal/robotpy-native-wpihal.pc",
     "//hal:robotpy-hal.generated_pkgcfg_files",
     "//ntcore:native/ntcore/robotpy-native-ntcore.pc",