| golang.org/x/tools/godoc/vfs | Package vfs defines types for abstract file system access and provides an implementation accessing the file system of the underlying OS. |
| path | Package path implements utility routines for manipulating slash-separated paths. |
| sync | Package sync provides basic synchronization primitives such as mutual exclusion locks. |
| time | Package time provides functionality for measuring and displaying time. |
| unicode/utf8 | Package utf8 implements functions and constants to support text encoded in UTF-8. |