cmd/go: fix import directory list for compilation
cmd/go: fix import directory list for compilation

This fixes the most annoying bug in the go command,
that 'go build' sometimes ignored packages it had just
rebuilt in favor of stale installed ones.

This part of the code needs more thought, but this small
change is an important improvement.

R=golang-dev, adg
CC=golang-dev
http://codereview.appspot.com/5531053

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