webtry: Organize Go code like we do everywhere else.

We have structured our Go code putting them in a go/ directory. We also
structured it to put "main package" in a file named main.go.

Running the following command line:

$ git grep "package main"

Will show exactly that.

BUG=None
R=humper@google.com

Review URL: https://codereview.chromium.org/755463002