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
http://codereview.appspot.com/5577058

dsriram/golang,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,tv42/old-go,gnanderson/go,gnanderson/go,tv42/old-go,dsriram/golang,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,tv42/old-go,apparentlymart/golang,gnanderson/go,apparentlymart/golang,dsriram/golang,dsriram/golang