Add an option to prefer the system package manager instead of directly downloading for download based packages

[aminya]

Some of the packages setup-cpp installs are installed by directly downloading the binaries (usually from GitHub) and storing them under ~/tool. This is flexible, clean, and reliable.

For example, setup-cpp can install all the versions of LLVM on any system without worrying about missing system dependencies. The uninstallation of them is as easy as deleting ~/llvm.

However, it might be useful to use the package manager of a system for local installations (apt, brew, and choco) to allow automatic updates. setup-cpp can add an option for preferring this method.

The packages that are installed by direct downloading:

• [ ] llvm
• [ ] ninja
• [ ] cmake

These packages that are not available on all package managers:

• [ ] kcov
• [ ] task

[aminya]

Also related to #42