Scan silently if there are no available kits

[xisui-MSFT]

In the offline discussion, there was two decisions around this experience:

1. If opening a CMake project and there’s an toolchain file entry in settings.json, then don’t do anything related to Kits.
2. If opening a CMake project and there is NO toolchain file entry in settings.json and if there is no kits.json, then silently start scanning for kits.

We finally decided not to implement (1) because a kit is a super set of a toolchain file, and although it's possible to detect multiple settings in settings.json instead of only the toolchain file entry, this is unnecessarily complicated.

[bobbrow]

This should also address #708