Fixes Issue 648 - Document template mixin instantiations.
Merge pull request #1485 from AndrejMitrovic/Fix648_2

Issue 648 - Document template mixin instantiations.
[Refactoring] Add Package::isPackageMod()
[Refactoring] Add Module::isPackageFile
[Refactoring] Tweak code block
Fix import issues

- Add package table in each scope, due to check which package/module name is accessible via imports.
- Renamed/Selective imports no longer create aliases for the introduced names.
- Merge imports in base class members to derived class
Add "FIXME" comment for issue 10604 in ScopeDsymbol::search
Fix existing tests to fit improved import mechanism
fix Issue 313 - [module] Fully qualified names bypass private imports
fix Issue 314 - [module] Static, renamed, and selective imports are always public
fix Issue 8667 - selective import breaks normal overload resolution
fix Issue 10128 - import statement in base class members should be private by default