internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/dtc.go

@@ -7,10 +7,10 @@ func (t Toolchain) newDTC() (pkg.Artifact, string) {
 		version  = "1.7.2"
 		checksum = "vUoiRynPyYRexTpS6USweT5p4SVHvvVJs8uqFkkVD-YnFjwf6v3elQ0-Etrh00Dt"
 	)
-	return t.NewPackage("dtc", version, pkg.NewHTTPGetTar(
-		nil, "https://git.kernel.org/pub/scm/utils/dtc/dtc.git/snapshot/"+
+	return t.NewPackage("dtc", version, newTar(
+		"https://git.kernel.org/pub/scm/utils/dtc/dtc.git/snapshot/"+
 			"dtc-v"+version+".tar.gz",
-		mustDecode(checksum),
+		checksum,
 		pkg.TarGzip,
 	), &PackageAttr{
 		// works around buggy test: