Fix bug with multiple GOPATH full package name resolving

This commit fixes the bug where GOPATH values that are longer than the input package name cause 'slice bounds out of range'  errors.

codegen/codegen.go

@@ -156,8 +156,12 @@ func fullPackageName(dir string, pkgName string) string {
 
 	for _, gopath := range filepath.SplitList(build.Default.GOPATH) {
 		gopath = filepath.Join(gopath, "src") + string(os.PathSeparator)
+		if len(gopath) > len(fullPkgName) {
+			continue
+		}
 		if strings.EqualFold(gopath, fullPkgName[0:len(gopath)]) {
 			fullPkgName = fullPkgName[len(gopath):]
+			break
 		}
 	}
 	return filepath.ToSlash(fullPkgName)

codegen/codegen_test.go

@@ -0,0 +1,31 @@
+package codegen
+
+import (
+	"go/build"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_fullPackageName(t *testing.T) {
+	origBuildContext := build.Default
+	defer func() { build.Default = origBuildContext }()
+
+	t.Run("gopath longer than package name", func(t *testing.T) {
+		build.Default.GOPATH = "/a/src/xxxxxxxxxxxxxxxxxxxxxxxx:/b/src/y"
+		var got string
+		ok := assert.NotPanics(t, func() { got = fullPackageName("/b/src/y/foo/bar", "bar") })
+		if ok {
+			assert.Equal(t, "/b/src/y/foo/bar", got)
+		}
+	})
+	t.Run("stop searching on first hit", func(t *testing.T) {
+		build.Default.GOPATH = "/a/src/x:/b/src/y"
+
+		var got string
+		ok := assert.NotPanics(t, func() { got = fullPackageName("/a/src/x/foo/bar", "bar") })
+		if ok {
+			assert.Equal(t, "/a/src/x/foo/bar", got)
+		}
+	})
+}