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.

Original Description

buildsystem: install auxiliary files to the correct location

Rather than requiring downstream distribution packagers to magically know that files hidden in /usr/share/cmake-$VERSION/ don't actually belong there, but need to be installed elsewhere to integrate into the system... just install them where they belong.

The current status quo is that each distro independently discovers that the package needs manual fixups, and independently copies files where they belong. Sometimes it takes multiple bug reports, one for each type of file. Many distros still don't provide the completion files, because they still never noticed they exist.

This brings the Auxiliary/ directory in line with existing methods for installing the cmake-gui.desktop and mime/packages/cmakecache.xml files.

After this change, distros can start removing their painful hacks and post-processing.

Fixes: #20522 (closed)
Topic-rename: aux-install-dest