[workspaces] Add support for sub-projects that use find_package

[AlessandroA]

When using conan workspaces, if a sub-project contains a find_package command that points to another sub-project, CMake will fail saying that it cannot find the package.

This is expected, because the package will be added as a sub-directory by the top-level CMakeLists.txt file generated by conan.

The presence of find_package commands is common in packages that use the conan cmake_paths generator, for example.

The recommended solution to this is to re-define find_package as a no-op for all sub-projects, which is what this PR implements. See #3302 for more details. Closes #3302

Note: System-level packages and other conan dependencies found through find_package are unaffected, because they are not in the list of conan workspace packages, hence find_package is not redefined for them.

Implements #3302.

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• I've opened another PR in the Conan docs repo to the develop branch, documenting this one. Also adding a description of the changes in the changelog.rst file. https://github.com/conan-io/docs

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[pleroux0]

This change will break projects that already redefine find_package. Making add_subdirectory and find_package work together is an open CMake issue.

A better solution might be to generate a dummy config file that can be found by find_package which results in a no-op. This alternative solution will IMO cause the least amount of surprises for projects using this. I also think that it's the CMake project's responsibility to do so as well as define <project>_DIR so that the dummy config is the config file found after add_subdirectory is called. If #4808 gets addressed then the problem can be solved from CMake's side and hopefully remain compatible if the Cmake issue gets resolved.

[lasote]

The workspaces feature has been refactored. There is still no find package support but some ideas about the right approach. Take a look at my comment here: #4523