Make salt.utils.which look for a path with .exe on Windows

If the argument passed to this function does not end in .exe and the
minion is running Windows, look for the path + '.exe', allowing both
'cmd.exe' and 'cmd' to be matched.
Optimization for .exe path check

The majority of minions will be running a non-Windows OS, so this tweak
keeps salt.utils.is_windows() from being run unless the non-.exe path
check fails.
Merge pull request #8121 from terminalmage/issue7904

Make salt.utils.which look for a path with .exe on Windows