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/libxml2.go

@@ -7,10 +7,9 @@ func (t Toolchain) newLibxml2() (pkg.Artifact, string) {
 		version  = "2.15.2"
 		checksum = "zwQvCIBnjzUFY-inX5ckfNT3mIezsCRV55C_Iztde5OnRTB3u33lfO5h03g7DK_8"
 	)
-	return t.NewPackage("libxml2", version, t.NewViaGit(
+	return t.NewPackage("libxml2", version, t.newTagRemote(
 		"https://gitlab.gnome.org/GNOME/libxml2.git",
-		"refs/tags/v"+version,
-		mustDecode(checksum),
+		"v"+version, checksum,
 	), &PackageAttr{
 		// can't create shell.out: Read-only file system
 		Writable: true,