fix 'failed to parse source' in mk-user-func when source contains module with dash-separated name

example:

```
$ plv8x -f 'plv8x.json patch_json(json, json[])=fast-json-patch:apply' -d test

/..../plv8x/node_modules/pg/lib/native/index.js:242
      if(error) throw error;
                      ^
failed to parse source fast-json-patch:apply
```