Improve Windows compatibility (#118)

- Use travis build matrix to test on Windows and macOS
- Make utility func compatible with host OS. Use filepath package to
  manipulate path strings for stripping $GOPATH from working directory.
- Delegate to go/build pkg for getting Go paths

Co-authored-by: Mike Lee <mike.lee@safeguardproperties.com>
Co-authored-by: Lucas Bremgartner <lucas@bremis.ch>

.travis.yml

@@ -6,11 +6,20 @@ branches:
   only:
     - master
 
-go:
-  - 1.11.x
-  - 1.12.x
-  - 1.13.x
-  - tip
+jobs:
+  include:
+    - os: linux
+      go: 1.11.x
+    - os: linux
+      go: 1.12.x
+    - os: linux
+      go: 1.13.x
+    - os: linux
+      go: tip
+    - os: osx
+      go: 1.13.x
+    - os: windows
+      go: 1.13.x
 
 before_install:
   - go get golang.org/x/lint/golint

pkg/moq/moq.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
+	"go/build"
 	"go/format"
 	"go/types"
 	"io"
@@ -355,18 +356,15 @@ func stripVendorPath(p string) string {
 	return strings.TrimLeft(path.Join(parts[1:]...), "/")
 }
 
-// stripGopath takes the directory to a package and remove the gopath to get the
-// canonical package name.
-//
-// taken from https://github.com/ernesto-jimenez/gogen
-// Copyright (c) 2015 Ernesto Jiménez
+// stripGopath takes the directory to a package and removes the
+// $GOPATH/src path to get the canonical package name.
 func stripGopath(p string) string {
-	for _, gopath := range gopaths() {
-		p = strings.TrimPrefix(p, path.Join(gopath, "src")+"/")
+	for _, srcDir := range build.Default.SrcDirs() {
+		rel, err := filepath.Rel(srcDir, p)
+		if err != nil || strings.HasPrefix(rel, "..") {
+			continue
+		}
+		return filepath.ToSlash(rel)
 	}
 	return p
 }
-
-func gopaths() []string {
-	return strings.Split(os.Getenv("GOPATH"), string(filepath.ListSeparator))
-}