gc: fix package quoting logic
gc: fix package quoting logic

The decision for when to say "hash/crc32".New instead of
crc32.New in an error was double-counting imports
from different packages or indirect imports, so it was
quoting even when there was no ambiguity.

R=ken2
CC=golang-dev
http://codereview.appspot.com/4645070

tv42/old-go,gnanderson/go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,ssrl/go,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,ssrl/go,gnanderson/go,gnanderson/go,gnanderson/go,tv42/old-go,apparentlymart/golang,dsriram/golang,dsriram/golang,apparentlymart/golang,tv42/old-go,ssrl/go,dsriram/golang,apparentlymart/golang,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,ssrl/go,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,ssrl/go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,ssrl/go,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,ssrl/go,tv42/old-go