CMake API from ign to gz

[chapulina]

• Part of Rename Ignition to Gazebo #698

Update gz-cmake's API to use gz_ instead of ign_, i.e. gz_install_all_headers, with a tick-tock cycle.

• cmake
  • Deprecate public ign functions in favor of gz gazebosim/gz-cmake#250
  • Deprecate ign_create_docs, ign_create_packages, ign_configure_build gazebosim/gz-cmake#253
  • Deprecate ign_build_tests, ign_get_libsources_and_unittests, ign_get_sources, ign_install_all_headers gazebosim/gz-cmake#254
  • Deprecate ign_manual_search, ign_create_packages, ign_create_core_library gazebosim/gz-cmake#255
  • Deprecate ign_configure_project, ign_*_executable, ign_import_target gazebosim/gz-cmake#257
  • Deprecate ign_find_package, ign_string_append, ign_build_error, ign_build_warning gazebosim/gz-cmake#258
  • [ign -> gz] Migrate more internal variables  gazebosim/gz-cmake#263
  • Deprecate ign_add_benckmarks in favor of gz_ gazebosim/gz-cmake#265
• utils
  • [ign -> gz] CMake functions gazebosim/gz-utils#65
• math
  • [ign -> gz] CMake functions gazebosim/gz-math#441
• tools
• plugin
  • [ign -> gz] CMake functions gazebosim/gz-plugin#89
• msgs
  • [ign -> gz] CMake functions gazebosim/gz-msgs#263
• common
  • [ign -> gz] CMake functions gazebosim/gz-common#370
• sdformat
  • [ign -> gz] CMake functions gazebosim/sdformat#1052
• transport
  • [ign -> gz] CMake functions gazebosim/gz-transport#322
• rendering
  • [ign -> gz] CMake functions gazebosim/gz-rendering#652
• physics
  • [ign -> gz] CMake functions gazebosim/gz-physics#358
• sensors
  • [ign -> gz] CMake functions gazebosim/gz-sensors#237
• gui
  • [ign -> gz] CMake functions gazebosim/gz-gui#422
• fuel-tools
  • [ign -> gz] CMake functions gazebosim/gz-fuel-tools#264
• sim (former ign-gazebo)
  • [ign -> gz] CMake functions gazebosim/gz-sim#1542
• launch
  • [ign -> gz] CMake functions gazebosim/gz-launch#171
• garden
  • [ign -> gz] CMake functions gazebosim/gz-garden#11

Before working on the most important changes, we'll open some PRs updating gz-cmake's internal APIs and docs, which should make future PRs easier to handle.