Go source outside $GOROOT.

If the argument is a path starting with / or ., disregard $GOROOT.
Also, disable the check for package name matching the directory,
which is counterproductive in this case.

Apologies for the violence to the code but we need some help documenting
Go code outside the standard repository.

R=gri
CC=golang-dev
https://golang.org/cl/201064