go: x/tools/cmd/gopls: file is not part of package

Reproduces with gopls@master and gopls@latest:

golang.org/x/tools/gopls v0.1.0
    golang.org/x/tools/gopls@v0.1.2-0.20190708203411-c8855242db9c h1:qFHl9H1pFOvam5c7rRNiSVPuASu5sS/VNZUj7KNAyGI=

and

$ gopls version
golang.org/x/tools/gopls v0.1.0
    golang.org/x/tools/gopls@v0.1.1 h1:mU/O8r53RjzdZfkqmFOX4iEt8PJxTPXyLdiuQNa27OE=

Steps to reproduce:

1. Create folder
2. Create a.go:

   package main
   
   type whatever struct{}
   

3. Create b.go:

   package main
   
   import "fmt"
   
   func main() {
    w := whatever{}
    fmt.Println("whatever", w)
   }
   

This compiles and runs fine, but in vscode with gopls I get a red squiggly over package with the error:

file:///usr/local/google/home/deklerk/workspace/interview-questions/foo/b.go is not part of a package LSP