cmd/go: reject vendor directories as module-mode build arguments unless -mod=vendor is set

[bcmills]

There is a TODO here yet to be done:

go/src/cmd/go/internal/modload/load.go

Lines 110 to 112 in b412fde

	if strings.HasPrefix(suffix, "/vendor/") {
	// TODO getmode vendor check
	pkg = strings.TrimPrefix(suffix, "/vendor/")

The effect of the missing check is that when -mod=vendor is not set, running go build or go test with an empty directory path beginning with ./vendor causes the go command to resolve and download the corresponding module, ignoring the contents of the vendor directory.

example.com$ go1.13beta1 mod init example.com
go: creating new go.mod: module example.com

example.com$ go1.13beta1 build ./vendor/golang.org/x/text/unicode
can't load package: package golang.org/x/text/unicode: unknown import path "golang.org/x/text/unicode": cannot find package

example.com$ mkdir -p vendor/golang.org/x/text/unicode

example.com$ go1.13beta1 build ./vendor/golang.org/x/text/unicode
go: finding golang.org/x/text v0.3.2
go: downloading golang.org/x/text v0.3.2
go: extracting golang.org/x/text v0.3.2

example.com$

(Found while investigating #32917.)

[bcmills]

Fixed by CL 185345.