go.tools: Expose filename rather than extension only, needed for new tour.
go.tools: Expose filename rather than extension only, needed for new tour.

R=adg, r
CC=golang-codereviews
https://codereview.appspot.com/43470050

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