[Refactoring] Add Package::isPackageMod()
[Refactoring] Add Module::isPackageFile
[Refactoring] Tweak code block
Add test case for issue 12359 before deprecation
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
Fundamental fix for Issue 12201
fix Issue 12359 - implicit overload merging with selective/renamed imports should be removed

Show deprecation message for the implicitly merged overload set, by using cross-module overload set mechanism.
Merge pull request #2256 from 9rnsr/fix_imports

Fix issue 313 & 314 implement more strict import mechanism