go: cmd/go: go list reports a mangled ImportPath for a relative path that is not a valid Go package

What version of Go are you using (go version)?

Using gotip as of 12/5/2019 because this workflow fails with an error using Go 1.13.4:

$ gotip version
go version devel +eeb319a Thu Dec 5 22:16:02 2019 +0000 darwin/amd64

What did you do?

$ mkdir ~/foo && cd $_
$ echo "module github.com/nmiyake/foo" > go.mod
$ gotip list -e -f {{.ImportPath}} .

What did you expect to see?

github.com/nmiyake/foo

What did you see instead?

_/Users/nmiyake/foo

When running with Go 1.13.4, see:

build .: cannot find module for path .

This is likely #35414, which has been fixed on tip.

Further information/context

When a Go file is added to the directory, the output is as expected:

$ echo "package main" > main.go
$ gotip list -e -f {{.ImportPath}} .
github.com/nmiyake/foo

The behavior previously worked as I would expect in GOPATH mode:

$ mkdir $GOPATH/src/github.com/nmiyake/foo && cd $_
$ gotip list -e -f {{.ImportPath}} .
github.com/nmiyake/foo

If the list command can find the go.mod file and determine the import path relative to that, it would be helpful if it returned the import path if Go files existed in the directory rather than the _ import path.