internal/pkg: implement http artifact

This is useful for downloading source tarballs from the internet.

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

internal/pkg/pkg_test.go

@@ -5,6 +5,7 @@ import (
 	"crypto/sha512"
 	"encoding/base64"
 	"io/fs"
+	"net/http"
 	"os"
 	"path/filepath"
 	"reflect"
@@ -281,6 +282,10 @@ func TestErrors(t *testing.T) {
 			}, 8)),
 		}, "got AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA" +
 			" instead of deadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeef"},
+
+		{"ResponseStatusError", pkg.ResponseStatusError(
+			http.StatusNotAcceptable,
+		), "the requested URL returned non-OK status: Not Acceptable"},
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {