internal/sync: move sync.Mutex implementation into new package
This CL refactors sync.Mutex such that its implementation lives in the
new internal/sync package. The purpose of this change is to eventually
reverse the dependency edge between internal/concurrent and sync, such
that sync can depend on internal/concurrent (or really, its contents,
which will likely end up in internal/sync).
The only change made to the sync.Mutex code is the frame skip count for
mutex profiling, so that the internal/sync frames are omitted in the
profile.
Change-Id: Ib3603d30e8e71508c4ea883a584ae2e51ce40c3f
Reviewed-on: https://go-review.googlesource.com/c/go/+/594056
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>