Draft: Use GNUInstallDirs for catkin packages

[mikepurvis]

An initial piece of this much larger proposal.

I don't yet have a REP or an end-to-end demonstration of what is being achieved here, but the changes as they stand should be a no-op, as I believe we're just using standard variables whose default values are what was previously hard-coded. See:

https://cmake.org/cmake/help/v3.0/module/GNUInstallDirs.html

I'm testing with CMake 3.16.3 (Ubuntu Focal), but the documentation referenced is 3.0 as that's what catkin's minimum version is currently set at, and I don't want to bump that unless there's good reason to do so.

---

I've confirmed that catkin itself still builds, of course, and I've spot-checked a handful of small packages early in the dependency tree, for example:

$ BASE=$(pwd)/test-install
$ cmake ~/roscpp_core/cpp_common \
  -DCMAKE_INSTALL_PREFIX=/. \
  -DCMAKE_INSTALL_DATADIR=${BASE}/out/share \
  -DCMAKE_INSTALL_BINDIR=${BASE}/out/bin \
  -DCMAKE_INSTALL_LIBEXECDIR=${BASE}/out/lib \
  -DCMAKE_INSTALL_LIBDIR=${BASE}/out/lib \
  -DCMAKE_INSTALL_INCLUDEDIR=${BASE}/dev/include \
  -DCATKIN_INSTALL_INTO_PREFIX_ROOT=0
-- Using CATKIN_DEVEL_PREFIX: /home/mikepurvis/build/devel
-- Using CMAKE_PREFIX_PATH: /home/mikepurvis/catkin-install;/opt/ros/noetic
-- This workspace overlays: /home/mikepurvis/catkin-install;/opt/ros/noetic
[...]
-- Build files have been written to: /home/mikepurvis/build

$ make install
[100%] Built target cpp_common
Install the project...
-- Install configuration: ""
-- Installing: /home/mikepurvis/build/test-install/out/lib/pkgconfig/cpp_common.pc
-- Installing: /home/mikepurvis/build/test-install/out/share/cpp_common/cmake/cpp_commonConfig.cmake
-- Installing: /home/mikepurvis/build/test-install/out/share/cpp_common/cmake/cpp_commonConfig-version.cmake
-- Installing: /home/mikepurvis/build/test-install/out/share/cpp_common/package.xml
-- Installing: /home/mikepurvis/build/test-install/out/lib/libcpp_common.so
-- Installing: /home/mikepurvis/build/test-install/dev/include
-- Installing: /home/mikepurvis/build/test-install/dev/include/ros
-- Installing: /home/mikepurvis/build/test-install/dev/include/ros/exception.h
-- Installing: /home/mikepurvis/build/test-install/dev/include/ros/datatypes.h
-- Installing: /home/mikepurvis/build/test-install/dev/include/ros/platform.h
-- Installing: /home/mikepurvis/build/test-install/dev/include/ros/types.h
-- Installing: /home/mikepurvis/build/test-install/dev/include/ros/cpp_common_decl.h
-- Installing: /home/mikepurvis/build/test-install/dev/include/ros/header.h
-- Installing: /home/mikepurvis/build/test-install/dev/include/ros/macros.h
-- Installing: /home/mikepurvis/build/test-install/dev/include/ros/debug.h

$ grep test-install test-install/out/share/cpp_common/cmake/cpp_commonConfig.cmake
  set(cpp_common_INSTALL_PREFIX /home/mikepurvis/build/test-install)
  set(cpp_common_INSTALL_BINDIR /home/mikepurvis/build/test-install/out/bin)
  set(cpp_common_INSTALL_LIBDIR /home/mikepurvis/build/test-install/out/lib)
  set(cpp_common_INSTALL_DATADIR /home/mikepurvis/build/test-install/out/share)
  set(cpp_common_INSTALL_INCLUDEDIR /home/mikepurvis/build/test-install/dev/include)
if(NOT "/home/mikepurvis/build/test-install/dev/include;/usr/include " STREQUAL " ")
  set(_include_dirs "/home/mikepurvis/build/test-install/dev/include;/usr/include")
    elseif("${idir} " STREQUAL "/home/mikepurvis/build/test-install/dev/include ")
      get_filename_component(include "${cpp_common_DIR}/../../..//home/mikepurvis/build/test-install/dev/include" ABSOLUTE)
    foreach(path /home/mikepurvis/build/test-install/out/lib;/home/mikepurvis/catkin-install/lib;/opt/ros/noetic/lib)

In the Nix case (which is what I actually care about here), the CMake and pkgconfig files can be safely relocated after build to the dev output (equivalent of a debian libfoo-dev package) and they will still be able to find their libraries since they have been generated with absolute paths to them.

Remaining tasks before merging this:

• [ ] Build at least to ros-base using catkin-tools, before and after this change, validate all generated CMake config changes are reasonable.
• [ ] The same, but with the devel- rather than installspaces.
• [ ] Decide if we like the <pkgname>_INSTALL_XXDIR naming (which is meant to ape what GNUInstallDirs looks like), or if it would make more sense to follow the <pkgname>_DIR convention already in use in a number of places (examples: ROS 1, ROS 2).
• [ ] Decide if finding of package.xml files will be part of this change or a separate one (they have a similar issue to libraries in that they are no longer findable relative to the CMake config, since they're part of the runtime split).
• [ ] Implement at least some of the companion changes in other packages to properly consume the new <pkg>_INSTALL_LIBDIR and <pkg>_INSTALL_DATADIR variables from their "extras" templates.

This is obviously a package that has to be modified pretty conservatively, so I'm open to suggestions for other means of validating this change and ensuring that it is safe.