Refactor: Add SmartJoin to handle IsAbs automatically

internal/filepathext/filepathext.go

@@ -0,0 +1,14 @@
+package filepathext
+
+import (
+	"path/filepath"
+)
+
+// SmartJoin joins two paths, but only if the second is not already an
+// absolute path.
+func SmartJoin(a, b string) string {
+	if filepath.IsAbs(b) {
+		return b
+	}
+	return filepath.Join(a, b)
+}