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

lattera/go.tools,lattera/go.tools,chenjianlong/go.tools,chenjianlong/go.tools,lattera/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools