visibility vs. accessibility of protected symbols

[Martin Nowak]

I made a pull request to fix private/package access checks.
https://github.com/D-Programming-Language/dmd/pull/669

Now I'm wondering if that goes into the right direction.

The shallow distinction of visibility vs. accessibility breaks the module  
system because
one can't safely add a private symbol without possibly affecting every  
dependent module.
Thus we're back at using underscore names to protect from that.

 From implementing access checks I found the right amount of checking is to  
do them
when resolving identifiers. Thus it's almost equal to visibility based  
protection.

We already hide symbols that are privately imported by an imported module.

martin