Add CMake alias targets for embedded projects
This PR adds CMake alias targets LibXslt::LibXslt
, LibXslt::LibExslt
, and LibXslt::xsltproc
for the LibXslt
, LibExslt
, and xsltproc
targets to better support using libxslt as an embedded project (e.g., FetchContent), matching the config files and find modules.