Consider installing all vendor packages into the same `/opt/gz_vendor` directory

[azeey]

This is just an idea/prototype right now, but it would be great if we could install all the vendor packages into the same directory. This would allow the gz tool to find all the libraries without having to set GZ_CONFIG_PATH since each library's yaml file will be placed in opt/gz_vendor/share/gz, which would be a default search path for the gz tool.

I have been experimenting with a hack where I call project(gz_vendor) right before ament_vendor like so

project(gz_vendor)
ament_vendor(${LIB_NAME_UNDERSCORE}_vendor
  SATISFIED ${${LIB_NAME_FULL}_FOUND}
  VCS_URL https://github.com/gazebosim/${GITHUB_NAME}.git
  VCS_VERSION ${LIB_VCS_VER}
  GLOBAL_HOOK
)
project(gz_utils_vendor)

It's reset back to the original project after ament_vendor since ament_package checks that the project name matches the project specified in the package.xml file.