Can't that new option simply be:

GOPATH=`pwd` go build

"GOPATH=pwd go build" doesn't work, except all my project code is placed under a "src" subfolder in my project folder. But I don't like to put a "src" subfolder in my project.

The point is, set GOPATH to the set of directories you want to search.

Or maybe I don't understand what you are asking for. Can you show a tiny complete example?

For example, this is a file under my project root:

// a.go
package main

import "b"

func main() {
  b.F()
}

And here is another package "b" in the vendor folder which is directly under the project root folder.

// vendor/b/b.go
package b

func F() {}

Then

$ GOPATH=`pwd` go build
a.go:3:8: cannot find package "b" in any of:
	/sdk//go/src/b (from $GOROOT)
	/myproject/src/b (from $GOPATH)

A bare "go build" will succeed, even if the package "b" is not vendored, but it is under GOPATH/src.
What I hope is if package "b" is not vendored, "go build -excludeExternalPackages" will fail, even if package "b" is under GOPATH/src.

It would be also ok if go tools support a "GOPATHSRC" env variable, so that I can use "GOPATHSRC=pwd go build" to build my project.

Damn it. It looks the "GOPATHSRC" env variable has really been supported.
Cool!

But it doesn't solve my problem. If package "b" is under GOPATH/src but not vendored, "GOPATHSRC=pwd go build" still will succeed.

my mistake, "GOPATHSRC=pwd" is a nonsense in my experiments. "GOPATHSRC" env variable is not supported yet.

Leaving on hold for package management.

Closed for the Go modules feature is mature now.