fix: differential package create with non local sources (#2444)

## Description

Currently there is a bug wherein differential will only work if the
source is a local file

## Checklist before merging

- [x] Test, docs, adr added or updated as needed
- [x] [Contributor Guide
Steps](https://github.com/defenseunicorns/zarf/blob/main/.github/CONTRIBUTING.md#developer-workflow)
followed

src/pkg/packager/creator/normal.go

@@ -48,12 +48,16 @@ type PackageCreator struct {
 	cfg *types.PackagerConfig
 }
 
-// NewPackageCreator returns a new PackageCreator.
-func NewPackageCreator(createOpts types.ZarfCreateOptions, cfg *types.PackagerConfig, cwd string) *PackageCreator {
-	if createOpts.DifferentialPackagePath != "" && !filepath.IsAbs(createOpts.DifferentialPackagePath) {
-		createOpts.DifferentialPackagePath = filepath.Join(cwd, createOpts.DifferentialPackagePath)
+func updateRelativeDifferentialPackagePath(path string, cwd string) string {
+	if path != "" && !filepath.IsAbs(path) && !helpers.IsURL(path) {
+		return filepath.Join(cwd, path)
 	}
+	return path
+}
 
+// NewPackageCreator returns a new PackageCreator.
+func NewPackageCreator(createOpts types.ZarfCreateOptions, cfg *types.PackagerConfig, cwd string) *PackageCreator {
+	createOpts.DifferentialPackagePath = updateRelativeDifferentialPackagePath(createOpts.DifferentialPackagePath, cwd)
 	return &PackageCreator{createOpts, cfg}
 }
 

src/pkg/packager/creator/normal_test.go

@@ -0,0 +1,58 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: 2021-Present The Zarf Authors
+
+// Package creator contains functions for creating Zarf packages.
+package creator
+
+import (
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestDifferentialPackagePathSetCorrectly(t *testing.T) {
+	type testCase struct {
+		name     string
+		path     string
+		cwd      string
+		expected string
+	}
+
+	absolutePackagePath, err := filepath.Abs(filepath.Join("home", "cool-guy", "zarf-package", "my-package.tar.zst"))
+	require.NoError(t, err)
+
+	testCases := []testCase{
+		{
+			name:     "relative path",
+			path:     "my-package.tar.zst",
+			cwd:      filepath.Join("home", "cool-guy", "zarf-package"),
+			expected: filepath.Join("home", "cool-guy", "zarf-package", "my-package.tar.zst"),
+		},
+		{
+			name:     "absolute path",
+			path:     absolutePackagePath,
+			cwd:      filepath.Join("home", "should-not-matter"),
+			expected: absolutePackagePath,
+		},
+		{
+			name:     "oci path",
+			path:     "oci://my-cool-registry.com:555/my-package.tar.zst",
+			cwd:      filepath.Join("home", "should-not-matter"),
+			expected: "oci://my-cool-registry.com:555/my-package.tar.zst",
+		},
+		{
+			name:     "https path",
+			path:     "https://neat-url.com/zarf-init-amd64-v1.0.0.tar.zst",
+			cwd:      filepath.Join("home", "should-not-matter"),
+			expected: "https://neat-url.com/zarf-init-amd64-v1.0.0.tar.zst",
+		},
+	}
+	for _, testCase := range testCases {
+		tc := testCase
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			require.Equal(t, tc.expected, updateRelativeDifferentialPackagePath(tc.path, tc.cwd))
+		})
+	}
+}