cmd/go: don't call ImportDir unnecessarily
This significantly speeds up the go tool on
slow file systems (or those with cold caches).
The following numbers were obtained using
an encrypted ext4 file system running on
Linux 3.7.9.
# Before
$ sudo sysctl -w 'vm.drop_caches=3'
$ time go list code.google.com/p/go.net/... | wc -l
9
real 0m16.921s
user 0m0.637s
sys 0m0.317s
# After
$ sudo sysctl -w 'vm.drop_caches=3'
$ time go list code.google.com/p/go.net/... | wc -l
9
real 0m8.175s
user 0m0.220s
sys 0m0.177s
R=rsc, r
CC=golang-dev
https://golang.org/cl/
7369044