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
When more than one value is given to the NAMES option this command by
default will consider one name at a time and search every directory for
it. Add a NAMES_PER_DIR option to tell this command to consider one
directory at a time and search for all names in it.
Previously this command was tested only implicitly as part of larger
tests. Add a RunCMake.find_program test to cover find_program cases
specifically and independently.
