fix: use portable path manipulation for choosing package names

This *should* fix the generated names of packages on windows, which currently take the form `C:\kdfjslfj\jkfsjldkfjsf\sjdkfldjf` (this was reported to me by someone attempting to generate yaegi symbols for one of my projects). That being said, I don't have a windows machine to test on.

cmd/yaegi/extract.go

@@ -7,7 +7,7 @@ import (
 	"fmt"
 	"io"
 	"os"
-	"path"
+	"path/filepath"
 	"strings"
 
 	"github.com/traefik/yaegi/extract"
@@ -50,7 +50,7 @@ func extractCmd(arg []string) error {
 	}
 
 	if name == "" {
-		name = path.Base(wd)
+		name = filepath.Base(wd)
 	}
 	ext := extract.Extractor{
 		Dest:    name,