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/gtk.go

@@ -10,10 +10,9 @@ func (t Toolchain) newGLib() (pkg.Artifact, string) {
 		version  = "2.88.0"
 		checksum = "T79Cg4z6j-sDZ2yIwvbY4ccRv2-fbwbqgcw59F5NQ6qJT6z4v261vbYp3dHO6Ma3"
 	)
-	return t.NewPackage("glib", version, t.NewViaGit(
+	return t.NewPackage("glib", version, t.newTagRemote(
 		"https://gitlab.gnome.org/GNOME/glib.git",
-		"refs/tags/"+version,
-		mustDecode(checksum),
+		version, checksum,
 	), &PackageAttr{
 		Paths: []pkg.ExecPath{
 			pkg.Path(fhs.AbsEtc.Append(