go.tools/importer: don't pass srcDir=os.Getwd to go/build.Import().
go.tools/importer: don't pass srcDir=os.Getwd to go/build.Import().

This makes imports independent of the process's working
directory.  (Perhaps this was a feature, but I haven't found a
situation in which it actually works.)

R=gri
CC=golang-dev
https://codereview.appspot.com/19420043

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