cmd/go: download test dependencies of all named packages

Fixes #8181.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/123870043

src/cmd/go/get.go

@@ -151,7 +151,9 @@ func download(arg string, stk *importStack, getTestDeps bool) {
 	}
 
 	// Only process each package once.
-	if downloadCache[arg] {
+	// (Unless we're fetching test dependencies for this package,
+	// in which case we want to process it again.)
+	if downloadCache[arg] && !getTestDeps {
 		return
 	}
 	downloadCache[arg] = true

src/cmd/go/test.bash

@@ -543,6 +543,17 @@ TEST go get cover
 unset GOPATH
 rm -rf $d
 
+TEST go get -t "code.google.com/p/go-get-issue-8181/{a,b}"
+d=$(TMPDIR=/var/tmp mktemp -d -t testgoXXX)
+export GOPATH=$d
+if ./testgo get -t code.google.com/p/go-get-issue-8181/{a,b}; then
+	./testgo list ... | grep go.tools/godoc > /dev/null || ok=false
+else
+	ok=false
+fi
+unset GOPATH
+rm -rf $d
+
 TEST shadowing logic
 export GOPATH=$(pwd)/testdata/shadow/root1:$(pwd)/testdata/shadow/root2