Detect and support pre-configured build directories

### Brief Issue Summary

Allow Usage of a pre-configured build directory.

### Expected:

1. Configure project via `cmake`/`ccmake`/`cmake-gui`
2. Open project source folder in VSC
3. (optional) Set the build directory in the configuration if non-default
4. CMake Tools picks up the CMakeCache.txt and determines the used variant and Kit or shows something useful if this is not possible automatically
5. Intellisense/Include directories etc. work (requires setting the compilation database and possibly a reconfigure with the `CMAKE_EXPORT_COMPILE_COMMANDS` define)

### Apparent Behavior:

- No sign that the CMakeCache is deteced
- Choosing a Kit matching the one used manually deletes the preconfigured build dir (see #392)

Start by tracing how CMake Tools discovers or selects a build directory and reads CMakeCache.txt after configuration through cmake, ccmake, or cmake-gui. Check the existing Kit selection behavior described in #392 and how the compilation database is configured. Done means a pre-configured directory is detected or clearly reported, is not deleted when its Kit is selected, and supports IntelliSense include information.

Written by the indexing model from the issue text.