cmd/go: remove $GOROOT as a go get target

Fixes issue 4186.

Back in the day, before the Go 1.0 release, $GOROOT was mandatory for building from source. Fast forward to now and $GOPATH is mandatory and $GOROOT is optional, and mainly used by those who use the binary distribution in uncommon places.

For example, most novices at least know about `sudo` as they would have used it to install the binary tarball into /usr/local. It is logical they would use the `sudo` hammer to `go get` other Go packages when faced with a permission error talking about the path they just had to use `sudo` on last time.

Even if they had read the documentation and set $GOPATH, go get will not work as expected as `sudo` masks most environment variables.

llucky(~) % ~/go/bin/go env | grep GOPATH
GOPATH="/home/dfc"
lucky(~) % sudo ~/go/bin/go env | grep GOPATH
GOPATH=""

This CL therefore proposes to remove support for using `go get` to download source into $GOROOT.

This CL also proposes an error when GOPATH=$GOROOT, as this is another place where new Go users can get stuck.

Further discussion: https://groups.google.com/d/topic/golang-nuts/VIg3fjHiHRI/discussion

R=rsc, adg, minux.ma
CC=golang-dev
https://codereview.appspot.com/6941058