Cmake yaml-cpp package name fix

[vaishakp]

Proposed changes

This PR changes the cmake name of the package in the build system from the default yaml-cpp to YAML_CPP, and sets it as an alias for the original yaml-cpp.

This ensures that an external installation of the library outside of the standard location can be found by cmake, through the default cmake env variable YAML_CPP_ROOT.

Upgrade instructions

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments

Currently, the package yaml-cpp is declared as itself in cmake setup files, with the hyphen. While this doesn't cause any issues when the library is installed in standard locations, the hyphen in the cmake package name causes a failure for yaml-cpp to be found when installed in non-standard locations, translating to a failure at the configure stage:

CMake Warning at cmake/SetupYamlCpp.cmake:4 (find_package):
  By not providing "Findyaml-cpp.cmake" in CMAKE_MODULE_PATH this project has
  asked CMake to find a package configuration file provided by yaml-cpp,
  but CMake did not find one.

  Could not find a package configuration file provided by yaml-cpp with any
  of the following names:

    yaml-cppConfig.cmake
    yaml-cpp-config.cmake

  Add the installation prefix of "yaml-cpp" to CMAKE_PREFIX_PATH or set
  "yaml-cpp_DIR" to a directory containing one of the above files.  If
  "yaml-cpp" provides a separate development package or SDK, be sure it has
  been installed.
Call Stack (most recent call first):
  CMakeLists.txt:162 (include)


CMake Error at cmake/SetupYamlCpp.cmake:8 (message):
  Could not find yaml-cpp.  If you want to fetch missing dependencies
  automatically, set SPECTRE_FETCH_MISSING_DEPS=ON.
Call Stack (most recent call first):
  CMakeLists.txt:162 (include)

-- Configuring incomplete, errors occurred!

This is due to the fact that cmake looks in the path <PackageName>_ROOT, which for this package is yaml-cpp_ROOT. As per IEEE Standards 1003.1-2001, this is not a valid environment variable, at least in UNIX and thus cannot be set.

[nilsvu]

Thanks for fixing this. It took me a bit of time to verify that this is the right thing to do, because changing names like this has the potential to break installations on other people's machines and clusters. But I'm now conviced that this is a good change 👍