[feature_request] display all visited cmake files in CMake project outline

### Brief Issue Summary

I recently migrated from using QtCreator to VSCode with this extension for my daily development due to VSCode having much better support for different languages (no more need to have 3 editors for editing python, C++ and Groovy code in the same project) and also because VSCode's C++ debugger is simply awesome - much better than the one provided by QtCreator.

However, I am missing one feature: in QtCreator when I configured my CMake project, its CMake outline displayed all CMake files that were visited (traversed) during the CMake configuration process. On the other hand, the CMake outline in VSCode displays only CMakeLists.txt files for each of my projects but does not display all *.cmake files that were included (either directly or transitively) from the main CMakeLists.txt. Adding support for that would make VSCode much more usable, especially because quick open feature would then be able to open CMake files that were used, even though if they are not directly part of the workspace, but are included from CMAKE_MODULE_PATH folders (as is case when using Conan package manager and Conan package that provides some CMake utilities).

### Expected:

CMake outline view should display all *.cmake files that were visited during the CMake configuration process.

### Apparent Behavior:

CMake outline view displays only source files for each project and main CMakeLists.txt file.

### Platform and Versions

- **Operating System**: MacOS 10.14.3
- **CMake Version**: 3.13.4
- **VSCode Version**: 1.31.1
- **CMake Tools Extension Version**: 1.1.3
- **Compiler/Toolchain**: Xcode 10.1

Start by locating the CMake project outline implementation and the configuration-process data used by the extension; the issue names no specific files or tests. Trace how visited and included CMake files are discovered, then verify that directly and transitively included *.cmake files appear in the outline and can be opened through quick open.

Written by the indexing model from the issue text.