Improve the experience around missing CMakeLists.txt files

I have a multi-root workspace, where one of the roots is for Python code, one is for CMake, and one is just for a reference to common headers that are not under the CMake root. I use the 1.3.0-beta of CMake Tools and it works fine. It would be even better if CMake Tools did not complain or otherwise react on roots that don't have CMakeLists.txt file.

_Originally posted by @astraujums in https://github.com/microsoft/vscode-cmake-tools/issues/271#issuecomment-581034133_

Reproduce the multi-root workspace described in the issue, with Python, CMake, and shared-header roots, then trace how CMake Tools discovers and reacts to workspace roots. Done means roots without a CMakeLists.txt file no longer trigger complaints or other CMake Tools reactions, while the CMake root continues to work.

Written by the indexing model from the issue text.