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After configuring the file `yaml-cpp-config.cmake.in`, the result ends up with
empty variables.  (see also the discussion in #774).

Rework this file and the call to `configure_package_config_file` according the
cmake documentation
(https://cmake.org/cmake/help/v3.22/module/CMakePackageConfigHelpers.html?highlight=configure_package_config#command:configure_package_config_file)
to overcome this issue and allow a simple `find_package` after install.

As there was some discussion about the place where to install the
`yaml-cpp-config.cmake` file, e.g. #1055, factor out the install location into
an extra variable to make it easier changing this location in the future.

Also untabify CMakeLists.txt in some places to align with the other code parts in this file.