Rename worker package cross_py_builder -> agent

The nested cross_py_builder package only contains the worker agent. Rename
it to agent/ to match its role and distinguish it from the ctrl package and
the PyPI distribution name. Internal imports are relative, so build_agent.py
and zeroconf_server.py are unaffected except for the cosmetic APP_NAME banner.

setup.py

@@ -7,7 +7,7 @@ setup(
     install_requires=open("requirements.txt").read().splitlines(),
     entry_points={
         "console_scripts": [
-            "cross-py-agent=cross_py_builder.build_agent:main",
+            "cross-py-agent=agent.build_agent:main",
             "cross-py-controller=ctrl.app:main",
         ],
     },