dotr should exec scripts

Consider script below

```scala
// script.scala
def truth(): Int = 42
println(truth())
```

In Scala, we can use command `scala` to exec it directly.

```
$ scala script.scala 
42
$ 
```

But `dotr` could not identify bare script, and it complains

```
$ dotr script.scala 
Error: Could not find or load main class script.scala
$ 
```