go/build: permit vendoring golang.org from std without breaking users

[bradfitz]

In #16333 a user reported that they were unable to vendor code which the Go standard library itself vendored.

To "fix" the problem for the 1.7 release, we renamed the "$GOROOT/src/vendor/golang.org" directory to "golang_org" with an underscore, so it wouldn't conflict with users.

This is slightly annoying for us to maintain in the stdlib.

We should probably fix it properly in go/build and/or cmd/go.

/cc @ianlancetaylor @rsc