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/gnu.go

@@ -888,10 +888,9 @@ func (t Toolchain) newGnuTLS() (pkg.Artifact, string) {
 		}
 	}
 
-	return t.NewPackage("gnutls", version, t.NewViaGit(
+	return t.NewPackage("gnutls", version, t.newTagRemote(
 		"https://gitlab.com/gnutls/gnutls.git",
-		"refs/tags/"+version,
-		mustDecode(checksum),
+		version, checksum,
 	), &PackageAttr{
 		Patches: []KV{
 			{"bootstrap-remove-gtk-doc", `diff --git a/bootstrap.conf b/bootstrap.conf
@@ -1146,10 +1145,9 @@ func (t Toolchain) newMPC() (pkg.Artifact, string) {
 		version  = "1.4.0"
 		checksum = "TbrxLiE3ipQrHz_F3Xzz4zqBAnkMWyjhNwIK6wh9360RZ39xMt8rxfW3LxA9SnvU"
 	)
-	return t.NewPackage("mpc", version, t.NewViaGit(
+	return t.NewPackage("mpc", version, t.newTagRemote(
 		"https://gitlab.inria.fr/mpc/mpc.git",
-		"refs/tags/"+version,
-		mustDecode(checksum),
+		version, checksum,
 	), &PackageAttr{
 		// does not find mpc-impl.h otherwise
 		EnterSource: true,