find_program() incorrectly prepended search path components
to absolute file paths, and incorrectly searched the current
working directory for files that contained no directory
separators.
* Replace calls cmFindProgramHelper::CheckDirectory(std::string())
with call of new method cmFindProgramHelper::CheckCompoundNames()
that checks for the presence of a directory separator in the
file name.
* Use cmSystemTools::CollapseCombinedPath rather than string
concatenation to properly combine absolute file names with
search path components.
* Add unit tests to verify corrections.
Fixes: #18044
