* Rename syncutil to sync.
* Add aliases to sync types.
* Replace existing usage of standard library sync package.
This will make it easier to swap out synchronization primitives. For example,
this will allow us to use primitives from github.com/sasha-s/go-deadlock to
check for lock ordering violations.
Updates #1472
PiperOrigin-RevId: 289033387
The former is needed for vfs.FileDescription to implement
memmap.MappingIdentity, and the latter is needed to implement getcwd(2).
PiperOrigin-RevId: 285051855