CMakeToolchain generator sets CMAKE_FRAMEWORK_PATH

[thorsten-klein]

Changelog: (Feature): CMakeToolchain generator sets CMAKE_FRAMEWORK_PATH to host package roots

Let cmake find_package() do its default search procedure instead of maintaining a list of directories as cpp_info.builddirs.

With this change users need to maintain only cmake module paths in cpp_info.builddirs (containing FindXXX.cmake).
cmake config paths (containing XXX-Config.cmake) do not need to be maintained anymore.
conan will add each host package root path automatically to CMAKE_FRAMEWORK_PATH, which allows cmake to find them automatically when doing its default search procedure
(Ref: https://cmake.org/cmake/help/latest/command/find_package.html#config-mode-search-procedure)

Statement from Kitware:

The CMAKE_FRAMEWORK_PATH variable will be used by find_* commands, on all platforms. 
The main difference is that by default, on non-Mac platforms, find_* will first look for 
CMAKE_PREFIX_PATH variable entries and then  CMAKE_FRAMEWORK_PATH. 
Order is inverted on Mac by default. 

Ref: #14334

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[thorsten-klein]

Ref: #14748

[thorsten-klein]

We are now setting environment variable CMAKE_FRAMEWORK_PATH which is also considered by cmake automatically. This solves our issue well. But nevertheless it would be benefitial for other users if the CMakeToolchain generator sets this cmake variable and works out of the box.

The big advantage what I see is:
if the CMAKE_FRAMEWORK_PATH is used, conan does not need to store any information about the builddirs anymore. cmake will automatically search within some default directories when doing find_package(), find_library() or find_path(). These are exactly the things within host context.
find_program() (which should only search in build context) is not affected by this variable

Ref: #14748

I guess that with this change also some cmake variables (which are currently set in conan_toolchain.cmake) could become obsolete. But this can be checked in a subsequent PR then.