runner: introduce runner abstraction

For now all it does is represent the name of the runner and what requirements
it has of the build pipeline.

Move some package definitions from the runner package set to where it belongs.

internal/runner/linux.go

@@ -0,0 +1,15 @@
+package runner
+
+type Linux struct {
+}
+
+func (r *Linux) String() string {
+	return "org.osbuild.linux"
+}
+
+func (p *Linux) GetBuildPackages() []string {
+	return []string{
+		"glibc",   // ldconfig
+		"systemd", // systemd-tmpfiles and systemd-sysusers
+	}
+}