Enable default module.ceylon (review only)

This pull request is just for review and discussion. Default module with name "default" with package "" seems to compile and run fine, with no detrimental effect on existing compilation or compilation of default module without module.ceylon:
```ceylon
module  { // no name, it is parsed to "default/unversioned"
    import ceylon.collection "1.1.0";
}

package;  // no name, it is parsed to ""
```
Although the compilers compiles the default module with module.ceylon and package.ceylon fine without imports, it does not add dependencies to the module annotation, although they are present in the typechecker. Some additional errors are also thrown.