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