cmd/go: solve ambiguity of get lp.net/project/foo

cmd/go: solve ambiguity of get lp.net/project/foo

This solves the ambiguity for "lp.net/project/foo". In these URLs,
"foo" could be a series name registered in Launchpad with its own
branch, and it could also be the name of a directory within the
main project branch one level up.

Solve it by testing if the series branch exists in Launchpad
and if it doesn't moving the root one level up.

R=rsc
CC=golang-dev
https://golang.org/cl/5577058