blueprint: move pipeline generation into its own package

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.

test/cases/disk_empty_blueprint.json

@@ -1,5 +1,6 @@
 {
   "compose": {
+    "distro": "fedora-30",
     "output-format": "partitioned-disk",
     "filename": "disk.img",
     "blueprint": {}