cmd/go: fix two bugs
Issue 3207 was caused by setting GOPATH=GOROOT.
This is a common mistake, so diagnose it at command start
and also correct the bug that it caused in get (downloading
to GOROOT/src/foo instead of GOROOT/src/pkg/foo).
Issue 3268 was caused by recognizing 'packages' that
had installed binaries but no source. This behavior is not
documented and causes trouble, so remove it. We can
revisit the concept of binary-only packages after Go 1.
Fixes #3207.
Fixes #3268.
R=golang-dev, r
CC=golang-dev
https://golang.org/cl/
5930044