Support for package manager dependency resolution

As a user of a CMake I might have 3rd party packages that would be found by find_package. 

CMake expects those packages to exist, with the use of a package manager they can be made available.

Such a package manager could be conan, vcpkg or cpp-pm, but also a system package manager like apt, pkgcon or choco.

Since CMake version 3.15 CMake provides the ability to inject a CMake script via CMAKE_PROJECT_INCLUDE_BEFORE variable.

In such a script one could make sure that if conanfile.txt is present than conan install would be called, same for vcpkg.json with vcpkg install.

The use case would be a Hello World application that uses fmt to print a message:

#include <fmt/core.h>

int main()
{
    fmt::print("Привет мир\n");
    return 0;
}

And the CMakeLists.txt code for it:

cmake_minimum_required(VERSION 3.15)

project(hello-world LANGUAGES CXX)

add_executable(hello-world main.cpp)

find_package(fmt REQUIRED)
target_link_libraries(hello-world PRIVATE fmt::fmt)

A conanfile.txt would look like this:

[requires]
fmt/7.1.3 
[generators]
cmake_find_package

A vcpkg.json would looke like this:

{
  "name": "hello-world",
  "version-string": "0.0.1",
  "dependencies": [
    "fmt"
  ]
}

As a user I expect a project to load, compile successfully when the right setup with CMAKE_PROJECT_INCLUDE_BEFORE is setup.

The custom script would be able to:

• load QtCreatorPackageManager.cmake script found in the source directory
• QT_CREATOR_SKIP_PACKAGE_MANAGER_SETUP would skip the whole auto-setup
• QT_CREATOR_SKIP_CONAN_SETUP would skip the conan auto-setup
• QT_CREATOR_SKIP_VCPKG_SETUP would skip the vcpkg auto-setup