* CMake: Add option to set the package install dir.

* CMake: Fix generated config.

- `YAML_CPP_SHARED_LIBS_BUILT` should not be set with a `PATH_VAR` as it
would always evaluate to true.
- `YAML_CPP_LIBRARIES` should used the exported target name including
the namespace, but `check_required_components` shouldn't.
- Use `CMAKE_CURRENT_LIST_DIR` to find the target file, instead of a
`PATH_VAR`. Package managers such as vcpkg move CMake configs after
installing.

* CI: Test the generated CMake package.

* CMake: Create add a deprecated yaml-cpp target.

This target is meant to provide compatibility with versions prior to
0.8.0.

* CMake: mark the yaml-cpp target as IMPORTED.

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Co-authored-by: Jesse Beder <jbeder+github@gmail.com>