set CMAKELIB_SRC_DIR as cache variable

[ClausKlein]

I renamed Index.cmake to CMakeLists.txt

Now it may be fetched and used with CPM.cmake module.

This offer the reuse of the cloned CMakeLib repo from ~/.cache/CPM ...

see usage example: https://github.com/ClausKlein/cpp_starter_project/blob/feature/use-cpm-cmake-module/CMakeLists.txt

TOO I fixed some minor problems found while testing the CMakeLib

[aminya]

This needs a rebase

[aminya]

I had many issues with using CMakeLists.txt for a CMake library. It broke Conan dependencies the last time I tried it.

[ClausKlein]

I had many issues with using CMakeLists.txt for a CMake library. It broke Conan dependencies the last time I tried it.

I will have a look at this

[aminya]

I reverted the cmake-format option as it was getting hard to follow. This should be a separate PR, and we should also update it in https://github.com/cpp-best-practices/cpp_starter_project.