Can't use code defined in another file and I can not use installed packages

I'm using Go 1.16 and GoLand 2020.3.2 on Windows 10.

GOPATH = C:\Users\user\go
GOROOT = C:\Users\user\go\go1.16

I have two problems.  The first is I can't use code defined in the same project but in a different file.  For example, the following code will raise an error:

# command-line-arguments
.\main.go:9:13: undefined: otherFunc

project/a.go

package main

func main() {
    otherFunc()
}

project/b.go

package main

func otherFunc() {
    return true
}

I am under the impression that Go does not require me to import "b.go" into "a.go" and that "a.go" has access to all the functions defined in "b.go".  Is this not the case or is there some configuration issue on my end?

Possibly related is the fact that I can not use any third party libraries installed from github.

If I type `import "github.com/julienschmidt/httprouter"`, GoLand will ask me to install the package. I click install and it does so successfully. The library is installed under `C:\Users\user\go\pkg\mod`. But GoLand looks for the package in `C:\Users\user\go\go1.16\src`.

What am I doing wrong?