go/build: go get errors on added sourcegraph dependency

[Skarlso]

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

$ go version
1.11.2 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
❯ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/gbrautigam/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/gbrautigam/go"
GOPROXY=""
GORACE=""
GOROOT="/Users/gbrautigam/gohome"
GOTMPDIR=""
GOTOOLDIR="/Users/gbrautigam/gohome/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/gbrautigam/gohome/src/github.com/golang/build/go.mod"
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/1n/k6w9b2g51hg_dhlh_c8q9q8r0000gn/T/go-build765178128=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

While export GO111MODULE=on.

git clone git@github.com:golang/build.git
cd build
go get ./...

What did you expect to see?

Everything is working fine.

What did you see instead?

❯ go get ./...
go: sourcegraph.com/sourcegraph/go-diff@v0.5.0: unrecognized import path "sourcegraph.com/sourcegraph/go-diff" (parse https://sourcegraph.com/sourcegraph/go-diff?go-get=1: no go-import meta tags ())
go: error loading module requirements

Commit that brought in that dependency happened 17 days ago here:
golang/build@56b8367

[Skarlso]

Interestingly, my company's IP was blacklisted on a CDN network which made all sorts of weirdness. Apparently it also affected go get.... :O I used a VPN and spoofed my IP and suddenly go get worked.