internal/rosa: wrapper around git helper

This results in much cleaner call site for the majority of use cases.

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

internal/rosa/netfilter.go

@@ -55,10 +55,9 @@ func (t Toolchain) newLibnftnl() (pkg.Artifact, string) {
 		version  = "1.3.1"
 		checksum = "91ou66K-I17iX6DB6hiQkhhC_v4DFW5iDGzwjVRNbJNEmKqowLZBlh3FY-ZDO0r9"
 	)
-	return t.NewPackage("libnftnl", version, t.NewViaGit(
+	return t.NewPackage("libnftnl", version, t.newTagRemote(
 		"https://git.netfilter.org/libnftnl",
-		"refs/tags/libnftnl-"+version,
-		mustDecode(checksum),
+		"libnftnl-"+version, checksum,
 	), &PackageAttr{
 		Env: []string{
 			"CFLAGS=-D_GNU_SOURCE",
@@ -98,10 +97,9 @@ func (t Toolchain) newIPTables() (pkg.Artifact, string) {
 		version  = "1.8.13"
 		checksum = "TUA-cFIAsiMvtRR-XzQvXzoIhJUOc9J2gQDJCbBRjmgmVfGfPTCf58wL7e-cUKVQ"
 	)
-	return t.NewPackage("iptables", version, t.NewViaGit(
+	return t.NewPackage("iptables", version, t.newTagRemote(
 		"https://git.netfilter.org/iptables",
-		"refs/tags/v"+version,
-		mustDecode(checksum),
+		"v"+version, checksum,
 	), &PackageAttr{
 		ScriptEarly: `
 rm \