We could not rely on .ui files when generating the ninja rules
for the generated UI header files. .ui files might be added to the
target sources but never processed by AUTOUIC afterward, since UI
header files are never included in a source code. Instead of adding
dependency rules based on the .ui files, this approach scans
non-generated source files for includes of the UI header files,
as AUTOUIC does. This gives the consistent set of UI header files
at configure time, that could be used to generate byproducts rules
for the AUTOUIC. Also, the path to the generated UI header file depends
not on the .ui file location but on the include line is used in source
files.

Fixes: #16776