gosweepdone is another anachronism from the time when the sweeper was
implemented in C. Rename it to "isSweepDone" for the modern era.

Change-Id: I8472aa6f52478459c3f2edc8a4b2761e73c4c2dd
Reviewed-on: https://go-review.googlesource.com/c/138658
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>