manifest: ensure uniqueness of pipeline names

Panic in case two pipelines with the same name are added to the
manifest. In the future we should deal with allocating fresh names,
but that is not yet needed.

internal/manifest/manifest.go

@@ -35,7 +35,11 @@ func New() Manifest {
 }
 
 func (m *Manifest) AddPipeline(p Pipeline) {
-	// TODO: enforce uniqueness - pipelines should have a back pointer to the manifest
+	for _, pipeline := range m.pipelines {
+		if pipeline.Name() == p.Name() {
+			panic("duplicate pipeline name in manifest")
+		}
+	}
 	m.pipelines = append(m.pipelines, p)
 	m.addPackages(p.getPackages())
 	m.addOSTreeCommits(p.getOSTreeCommits())