osbuild: use osbuild_cli directly as main entry

Instead of having a another indirection via `main_cli`, directly
use `osbuild_cli` in as main function in `__main__.py`. Also use
that in as the entry point for the generated `osbuild` executable.
Change `osbuild_cli` to be self-contained, i.e. it directly uses
`sys.argv` and `sys.exit`.

setup.py

@@ -11,7 +11,7 @@ setuptools.setup(
     ],
     entry_points={
         "console_scripts": [
-            "osbuild = osbuild.main_cli:main_cli"
+            "osbuild = osbuild.main_cli:osbuild_cli"
         ]
     },
 )