Vim autocompletion respects the $GOPATH variable and does not
ignore dashes ('-'), dots ('.') and underscores ('_') like found
in many remote packages.
Environment variable $GOROOT is determined by calling
'go env GOROOT' instead of relying on the user's environment
variables.
Fixes#3876Fixes#3882
R=golang-dev, franciscossouza, dsymonds, minux.ma
CC=golang-dev
https://golang.org/cl/6443151