osbuild: extract CLI to prepare for additional entrypoints

This extracts the CLI entrypoint into `main_cli.py` and prepares the
codebase for the introduction of additional entrypoints. This should
not contain any functional changes.

The idea behind this is to add `main_api.py` (and maybe more in the
future), which will be similar to `main_cli.py` but contain the
`osbuild-api` entrypoint. This will make all entrypoints nicely symetric
and the only difference will be `setup.py` selecting the right
entrypoint for each executable, as well as `__main__.py` selecting the
entrypoint for the module itself (which we will keep to the CLI for
compatibility).

osbuild/__init__.py

@@ -1,3 +1,13 @@
+"""OSBuild Module
+
+The `osbuild` module provides access to the internal features of OSBuild. It
+provides parsers for the input and output formats of osbuild, access to shared
+infrastructure of osbuild stages, as well as a pipeline executor.
+
+The utility module `osbuild.util` provides access to common functionality
+independent of osbuild but used across the osbuild codebase.
+"""
+
 
 from .pipeline import Assembler, load, load_build, Pipeline, Stage