go.tools/importer: fix classic closing-over-induction-variable gotcha.
go.tools/importer: fix classic closing-over-induction-variable gotcha.

Now we actually prefetch all n packages, instead of
prefetching the last one n times.  This yields a further 22%
improvement, which is more like what I was hoping for.

It makes me so sad that Go reproduced one of the best-known
mistakes of JavaScript.  D'oh!

R=crawshaw
CC=golang-dev
https://codereview.appspot.com/13379046

8limbs/gotools,skyportsystems/go.tools,webfd/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,8limbs/gotools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,8limbs/gotools,d0f/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,skyportsystems/go.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,d0f/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,webfd/go-zh.tools,bryanxu/go-zh.tools