Fix extra v in golang tags

[mxr]

While running the mirror I saw the tags had an extra v:

$ git tag | head -n5
vv0.1.0
vv0.1.1
vv0.2.0
vv0.2.1
vv0.3.0

There is a v in the version identifier returned by the Golang proxy API. I guess another person tried to work around it in a different way by stripping/adding a v. Instead, to fix this I stripped the v when we get the versions but add it back for additional_dependencies. I used try-repo to test the hook still works

[asottile]