cmd/go: validate module proxy URLs received from go-get=1 queries

What version of Go are you using (go version)?

Go tip at CL 170879 or later.

Does this issue reproduce with the latest release?

no

What operating system and processor architecture are you using (go env)?

GOARCH="amd64" GOBIN="" GOCACHE=SHOWS_CORRECTLY GOEXE="" GOFLAGS="" GOHOSTARCH="amd64" GOHOSTOS="darwin" GOOS="darwin" GOPATH=SHOWS_CORRECTLY GOPROXY="" GORACE="" GOROOT="/usr/local/Cellar/go/1.12.2/libexec" GOTMPDIR="" GOTOOLDIR="/usr/local/Cellar/go/1.12.2/libexec/pkg/tool/darwin_amd64" GCCGO="gccgo" CC="clang" CXX="clang++" CGO_ENABLED="1" GOMOD=SHOWS_CORRECTLY CGO_CFLAGS="-g -O2" CGO_CPPFLAGS="" CGO_CXXFLAGS="-g -O2" CGO_FFLAGS="-g -O2" CGO_LDFLAGS="-g -O2" PKG_CONFIG="pkg-config" GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/j0/qxnj38ld075bj80p9hs2dqlw0000gn/T/go-build144819988=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

on travis, when running tests against master, modules are unable to be downloaded. i am unsure if this is because of travis or if it can be replicated elsewhere

What did you expect to see?

successful download of modules

What did you see instead?

go: github.com/golang-migrate/migrate/v4@v4.3.0 requires
	github.com/fsouza/fake-gcs-server@v1.5.0 requires
	cloud.google.com/go@v0.36.0 requires
	golang.org/x/build@v0.0.0-20190111050920-041ab4dc3f9d requires
	dmitri.shuralyov.com/app/changes@v0.0.0-20180602232624-0a106ad413e3: Get https:///dmitri.shuralyov.com/app/changes/@v/v0.0.0-20180602232624-0a106ad413e3.info: http: no Host in request URL
The command "go mod download" failed and exited with 1 during .

specifically, the 3x slashes (/) in the url. removing the slash resolves correctly