go.tools/cmd/doc: make filename logic separator-agnostic
go.tools/cmd/doc: make filename logic separator-agnostic

(Speculative fix for broken cmd/godoc test on Windows.)

LGTM=gri
R=gri
CC=golang-codereviews
https://codereview.appspot.com/90090043

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