imports: use chan struct{} for disk semaphore
imports: use chan struct{} for disk semaphore

Update issue 7747

LGTM=bradfitz
R=bradfitz, iant
CC=golang-codereviews
https://codereview.appspot.com/86280043

chenjianlong/go.tools,karmakaze/go.tools,karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools,karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools