Aux: Install editor and bash files to more natural locations

The vim, emacs, and bash support files are not internal CMake resources
and so do not belong under `CMAKE_DATA_DIR`.  Move them over to proper
places under the `CMAKE_XDGDATA_DIR` as we do already for cmake-gui
desktop files and `cmake.m4`.

Fixes: #20522

Auxiliary/CMakeLists.txt

-install(DIRECTORY vim/indent vim/syntax DESTINATION ${CMAKE_DATA_DIR}/editors/vim)
-install(FILES cmake-mode.el DESTINATION ${CMAKE_DATA_DIR}/editors/emacs)
+install(DIRECTORY vim/indent vim/syntax DESTINATION ${CMAKE_XDGDATA_DIR}/vim/vimfiles)
+install(FILES cmake-mode.el DESTINATION ${CMAKE_XDGDATA_DIR}/emacs/site-lisp)
 install(FILES cmake.m4 DESTINATION ${CMAKE_XDGDATA_DIR}/aclocal)
 add_subdirectory (bash-completion)

Auxiliary/bash-completion/CMakeLists.txt

-# Always install completion file in local dir
-# in order to be sure to always be able to install
-# in a local user directory rooted in a single directory.
-# packager should either patch that out or
-# add symlinks to the files in appropriate places
-#  /etc/bash_completion.d/
-#  DATADIR/completions (may be /usr/share/<package>/completions
-install(FILES cmake cpack ctest DESTINATION ${CMAKE_DATA_DIR}/completions)
+# We need to integrate into the system install, or this will silently fail to
+# accomplish anything at all, and packagers won't even know it exists. Use the
+# `<sharedir>/bash-completion/completions/` hierarchy by default, rooted in
+# CMake's XDGDATA_DIR definition of the sharedir. This works with installation
+# to `/usr` or `/usr/local` (or any prefix which bash-completion is configured
+# with) as well as a simple installation by a local user into their home
+# directory *if* the prefix is `$HOME/.local` since `.local/share/` is part of
+# the bash-completion search path too.
+# For more complex installations, packagers can set CMAKE_BASH_COMP_DIR to
+# another system location.
+
+set(CMAKE_BASH_COMP_DIR_DEFAULT ${CMAKE_XDGDATA_DIR}/bash-completion/completions)
+if (NOT CMAKE_BASH_COMP_DIR)
+  set(CMAKE_BASH_COMP_DIR "${CMAKE_BASH_COMP_DIR_DEFAULT}")
+endif()
+install(FILES cmake cpack ctest DESTINATION ${CMAKE_BASH_COMP_DIR})