go/loader: convert directory separators to slash when enumerating packages.
go/loader: convert directory separators to slash when enumerating packages.

(On Windows the two are not the same.)

Fixes issue 7189.

Also: remove exception for code.google.com subtree; this was
an artifact of my unusual setup.

LGTM=gri
R=alex.brainman, gri
CC=golang-codereviews
https://codereview.appspot.com/70060048

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