Add build_modules to cpp_info and manage CMake modules

[danimtb]

Changelog: Feature: Add cpp_info.build_modules to manage build system modules like additional CMake functions in packages
Docs: conan-io/docs#1465

• Refer to the issue that supports this Pull Request: closes CMake generators including .cmake in builddirs #5798
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

^{Note: By default this PR will skip the slower tests and will use a limited set of python versions. Check here how to increase the testing level by writing some tags in the current PR body text.}

[danimtb]

The property is needed in order to convert the relative paths to absolute

[danimtb]

Worked on a real example using the pybind11 library. Changes required can be found here: https://github.com/conan-community/conan-pybind11/compare/release/2.3.0...danimtb:release/2.3.0_build_modules?expand=1

The "cannonical" way of using this library is described here: https://pybind11.readthedocs.io/en/stable/compiling.html#building-with-cmake

[danimtb]

Added docs

[memsharded]

I am not sure this fulfills the specification we had in mind, with 2 different fields:

• cpp_info.build_modules: will inject and load those files (can assume they exists)
• cpp_info.build_modules_paths: will add those paths to the build system search path

The later was necessary for functionality that users don't want to auto-load, but want to explicitly include() in their CMakeLists.txt. I think it is not necessary to filter empty directories in the build_modules_paths. This one can also be used to define paths in case the automatically injected ones transitively include other .cmake scripts in the package.

[danimtb]

Changes done!