Restore GOPATH-mode get (revert de4d503)

src/cmd/go/testdata/script/get_issue16471.txt

@@ -0,0 +1,23 @@
+[!net:rsc.io] skip
+[!net:github.com] skip
+[!git] skip
+
+env GO111MODULE=off
+
+cd rsc.io/go-get-issue-10952
+
+exec git init
+exec git add foo.go
+exec git config user.name Gopher
+exec git config user.email gopher@golang.org
+exec git commit -a -m 'initial commit'
+exec git remote add origin https://github.com/golang/go-get-issue-10952
+
+exec git status
+
+! go get -x -u rsc.io/go-get-issue-10952
+stderr '^package rsc.io/go-get-issue-10952: rsc\.io/go-get-issue-10952 is a custom import path for https://github.com/rsc/go-get-issue-10952, but .* is checked out from https://github.com/golang/go-get-issue-10952$'
+
+-- rsc.io/go-get-issue-10952/foo.go --
+// Junk package to test go get.
+package foo