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).
421414ef0b