Ensure module name conforms to file name

[smarr]

Currently, we don't have support for any top-level expressions.
And it happens from time to time that the top-level module definition (i.e. the main class) in a file does not have the same name as the file.

This can be confusing, because of copy/paste mistakes for instance.
We should check that. Don't think there is any use for allowing something else.
In Java, it is a little annoying, but gives a good consistency.

This is a restriction that's not useful if we would have multiple modules or other expressions, for instance for scripting support of course.