You can already do "using ClassX", but yeah, the package management story in C++ isn't great.
There's a proposal for modules in C++[1], which should help, but as far as I know that's not going to be in C++17. Clang already has a (different) version implemented as well[2].
[1] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n446... [2] http://clang.llvm.org/docs/Modules.html
Its probably a bit easier with C as it has a common ABI, and C# as it only has one tool chain, and not multiple toolchains from multiple vendors like C++.
Modules will help here further.
This is the one thing I really like about .NET. C++ would be so much more accessible to beginners if it was easier to integrate other libraries. Even integrating something as common as boost is quite hard with Visual Studio on Windows.
A C++ implementation could provide translated units which encapsulate not only compiled code, but also contain compiled class declarations, such that these units can just be used without any preprocessing, tokenizing or parsing. That would be outside the language.
The C++ language as it stands is defined in terms of a textual representation: everything proceeds from translation units being scanned into preprocessing tokens and so forth.
To use a compiled class declaration, you would still need to see the source code somewhere, in some form, in some documentation. Otherwise, how would you know that the frobozz class has a freakout method which accepts two int parameters.