internal/rosa: helpers for common url formats

This cleans up call site of NewPackage.

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

internal/rosa/unzip.go

@@ -22,11 +22,11 @@ make -f unix/Makefile generic1
 mkdir -p /work/system/bin/
 mv unzip /work/system/bin/
 `, pkg.Path(AbsUsrSrc.Append("unzip"), true, t.NewPatchedSource(
-		"unzip", version, pkg.NewHTTPGetTar(
-			nil, "https://downloads.sourceforge.net/project/infozip/"+
+		"unzip", version, newTar(
+			"https://downloads.sourceforge.net/project/infozip/"+
 				"UnZip%206.x%20%28latest%29/UnZip%20"+version+"/"+
 				"unzip"+strings.ReplaceAll(version, ".", "")+".tar.gz",
-			mustDecode(checksum),
+			checksum,
 			pkg.TarGzip,
 		), false,
 	))), version