Supplemental Go synchronization libraries

 This package provides supplemental Go libraries (golang.org/x/sync) that
 were not included in the main distribution. It provides Go concurrency
 primitives in addition to the ones provided by the language ro the "sync"
 and "sync/atomic" packages.
 .
 Provides:
   - singleflight: provides a duplicate function call suppression mechanism
   - errgroup: provides synchronization, error propagation, and Context
               cancellation for groups of goroutines working on subtasks
   - semaphore: provides a weighted semaphore implementation
   - syncmap: provides a concurrent map implementation