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/libxslt.go

@@ -7,10 +7,9 @@ func (t Toolchain) newLibxslt() (pkg.Artifact, string) {
 		version  = "1.1.45"
 		checksum = "MZc_dyUWpHChkWDKa5iycrECxBsRd4ZMbYfL4VojTbung593mlH2tHGmxYB6NFYT"
 	)
-	return t.NewPackage("libxslt", version, t.NewViaGit(
+	return t.NewPackage("libxslt", version, t.newTagRemote(
 		"https://gitlab.gnome.org/GNOME/libxslt.git",
-		"refs/tags/v"+version,
-		mustDecode(checksum),
+		"v"+version, checksum,
 	), nil, &MakeHelper{
 		Generate: "NOCONFIGURE=1 ./autogen.sh",