AUTOUIC: Fix internal paths of generated ui_foo.h files

For every file foo.ui we generate a ui_foo.h file in
${target}_autogen/include or ${target}_autogen/include_$<CONFIG> in the
multi-config case.  Even .ui files in subdirectories are handled this
way.  That means, .ui files with the same base name will conflict in a
target.

However, for .ui files in subdirectories we added generated sources with
the nonexistent path ${target}_autogen/include/subdir/ui_foo.h.  This
patch fixes that.

Also, CMake will now yield an error if a target has multiple .ui files
with the same base name.

Fixes #23523