go: go/build: Import doesn't work as expected with modules

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

$ go version
go version go1.12.4 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

$ go env
GOARCH=“amd64”
GOBIN=“”
GOCACHE=“/Users/maxgale/Library/Caches/go-build”
GOEXE=“”
GOFLAGS=“”
GOHOSTARCH=“amd64”
GOHOSTOS=“darwin”
GOOS=“darwin”
GOPATH=“/Users/maxgale/go”
GOPROXY=“”
GORACE=“”
GOROOT=“/usr/local/go”
GOTMPDIR=“”
GOTOOLDIR=“/usr/local/go/pkg/tool/darwin_amd64”
GCCGO=“gccgo”
CC=“clang”
CXX=“clang++”
CGO_ENABLED=“1”
GOMOD=“/Users/maxgale/Dev/my-test-package/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/pd/4cgwkjzx609_9yb4xdb2lfp80000gp/T/go-build151436703=/tmp/go-build -gno-record-gcc-switches -fno-common”

What did you do?

	packageName := "github.com/dimes/my-test-package"
	imported, err := build.Default.Import(packageName, ".", build.FindOnly)
	if err != nil {
		return nil, errors.Wrapf(err, "Error importing package %s", packageName)
	}

	packages, err := parser.ParseDir(fileSet, imported.Dir, nil, 0)
	if err != nil {
		return nil, errors.Wrapf(err, "Error parsing package %s", packageName)
	}

	for _, astPkg := range packages {
		var files []*ast.File
		for _, file := range astPkg.Files {
			files = append(files, file)
		}

		info := &types.Info{
			Defs: make(map[*ast.Ident]types.Object),
		}

		conf := types.Config{
			Importer: importer.ForCompiler(fileSet, "source", nil),
		}

		_, err := conf.Check(packageName, fileSet, files, info)
		if err != nil {
			panic(err) // Panics here
		}
	}

What did you expect to see?

I expected no error from conf.Check

What did you see instead?

panic: /Users/maxgale/Dev/my-test-package/bindings/bindings.go:6:2: could not import github.com/dimes/my-test-package/prototype (type-checking package "github.com/dimes/my-test-package/prototype" failed (/Users/maxgale/Dev/my-test-package/prototype/basic.go:3:8: could not import <omitted> could not import github.com/golang/protobuf/ptypes/wrappers (type-checking package "github.com/golang/protobuf/ptypes/wrappers" failed (/Users/maxgale/go/pkg/mod/github.com/golang/protobuf@v1.2.0/ptypes/wrappers/wrappers.pb.go:6:14: could not import github.com/golang/protobuf/proto (cannot find package "github.com/golang/protobuf/proto" in any of:
	/usr/local/go/src/github.com/golang/protobuf/proto (from $GOROOT)
	/Users/maxgale/go/src/github.com/golang/protobuf/proto (from $GOPATH))))))))))))))))

However, if from the command line, I run go list -f '{{ .Dir }}' github.com/golang/protobuf/proto I get /Users/maxgale/go/pkg/mod/github.com/golang/protobuf@v1.3.1/proto. Another peculiarity is that the omitted packages are pulled from Github Enterprise repos.