Introduce the `distro` package, which contains an interface for OS
implementations. Its main purpose is to convert a blueprint to a
distro-specific pipeline.
Also introduce the `distro/fedora30` package. It is the first
implementation of the distro interface. Most of its code has been copied
with minimal modifications from the blueprint package.
The `blueprint` package is now back to serving a single purpose:
representing a weldr blueprint. It does not depend on the `pipeline`
package anymore.
Change osbuild-composer and osbuild-pipeline to use the new API,
hard-coding "fedora-30". This looks a bit weird now, but is the same
behavior as before.
All test cases now also take an "distro" key in the "compose" object.
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