Import package by files if outside of GOPATH

This is for example the case, if a mock is generated for an external
interface while go modules are enabled.

Fixes: #79

pkg/moq/moq.go

@@ -10,6 +10,7 @@ import (
 	"go/token"
 	"go/types"
 	"io"
+	"io/ioutil"
 	"os"
 	"path"
 	"path/filepath"
@@ -234,7 +235,19 @@ func pkgInfoFromPath(src string) (*loader.PackageInfo, error) {
 		ParserMode: parser.SpuriousErrors,
 		Cwd:        src,
 	}
-	conf.Import(pkgFull)
+	if strings.HasPrefix(pkgFull, string(filepath.Separator)) {
+		files, err := ioutil.ReadDir(pkgFull)
+		if err != nil {
+			return nil, err
+		}
+		for _, file := range files {
+			if !file.IsDir() && strings.HasSuffix(file.Name(), ".go") && !strings.HasSuffix(file.Name(), "_test.go") {
+				conf.CreateFromFilenames(abs, file.Name())
+			}
+		}
+	} else {
+		conf.Import(pkgFull)
+	}
 	lprog, err := conf.Load()
 	if err != nil {
 		return nil, err