x/tools/internal/lsp: gopls breaks with C dependencies

[marwan-at-work]

Gopls (on vscode) fails to compile and autocomplete when using gocv.io/x/gocv as a dependency in your program.

To reproduce:

1. Follow the installation instructions here: https://gocv.io/getting-started/macos/
2. Create a Go program outside GOPATH with a go.mod file
3. Paste the following into main.go:

package main

import (
	"gocv.io/x/gocv"
)

func main() {
	webcam, _ := gocv.VideoCaptureDevice(0)
	window := gocv.NewWindow("Hello")
	img := gocv.NewMat()

	for {
		webcam.Read(&img)
		window.IMShow(img)
		window.WaitKey(1)
	}
}

From a terminal: run go run main.go and notice that it works by opening your camera.

However, notice that in VSCode, Gopls is unable to find any of the functions in this file:

Gopls version:

commit aa71c3f32488bfee7774668abdf1ae045e013079 (HEAD -> master, origin/master, origin/HEAD)
Author: Rebecca Stambler <[email protected]>
Date:   Tue May 28 18:13:06 2019 -0400

    internal/lsp: set env to os.Environ to start

[marwan-at-work]

duplicate of #31561

will re open if 31561 is solved and this is still an issue.