go.tools/oracle: fix bug in reduceScope on ad-hoc main packages.
go.tools/oracle: fix bug in reduceScope on ad-hoc main packages.

LGTM=crawshaw
R=crawshaw
CC=golang-codereviews
https://codereview.appspot.com/77450048

t-yuki/go.tools.old,t-yuki/go.tools.old,t-yuki/go.tools.old