cmd/dist: simplify work list functions
There are no uses of addCmd, so delete it. The only use of bgDirCmd is
dirCmd, so inline it. Now the only function that interacts with the
work queue is registerTest and dist's "background commands" are used
exclusively in goTest.bgCommand and registerTest (which calls
goTest.bgCommand).
For #37486.
Change-Id: Iebbb24cf9dbee45f3975fe9504d858493e1cd947
Reviewed-on: https://go-review.googlesource.com/c/go/+/494956
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>