internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/make.go

@@ -20,9 +20,9 @@ cd "$(mktemp -d)"
 	--disable-dependency-tracking
 ./build.sh
 ./make DESTDIR=/work install check
-`, pkg.Path(AbsUsrSrc.Append("make"), false, pkg.NewHTTPGetTar(
-		nil, "https://ftpmirror.gnu.org/gnu/make/make-"+version+".tar.gz",
-		mustDecode(checksum),
+`, pkg.Path(AbsUsrSrc.Append("make"), false, newTar(
+		"https://ftpmirror.gnu.org/gnu/make/make-"+version+".tar.gz",
+		checksum,
 		pkg.TarGzip,
 	))), version
 }