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.

internal/distro/distro.go

@@ -0,0 +1,48 @@
+package distro
+
+import (
+	"github.com/osbuild/osbuild-composer/internal/blueprint"
+	"github.com/osbuild/osbuild-composer/internal/pipeline"
+)
+
+type Distro interface {
+	// Returns a sorted list of the output formats this distro supports.
+	ListOutputFormats() []string
+
+	// Returns the canonical filename and MIME type for a given output
+	// format
+	FilenameFromType(outputFormat string) (string, string, error)
+
+	// Returns an osbuild pipeline that generates an image in the given
+	// output format with all packages and customizations specified in the
+	// given blueprint. `outputFormat` must be one returned by
+	// ListOutputFormats().
+	Pipeline(b *blueprint.Blueprint, outputFormat string) (*pipeline.Pipeline, error)
+}
+
+// An InvalidOutputFormatError is returned when a requested output format is
+// not supported. The requested format is included as the error message.
+type InvalidOutputFormatError struct {
+	Format string
+}
+
+func (e *InvalidOutputFormatError) Error() string {
+	return e.Format
+}
+
+var registered = map[string]Distro{}
+
+func New(name string) Distro {
+	distro, ok := registered[name]
+	if !ok {
+		panic("unknown distro: " + name)
+	}
+	return distro
+}
+
+func Register(name string, distro Distro) {
+	if _, exists := registered[name]; exists {
+		panic("a distro with this name already exists: " + name)
+	}
+	registered[name] = distro
+}