cmFindProgramCommand: Re-implement search using more flexible approach

Avoid using KWSys SystemTools::FindProgram because it does much more
than we actually need for find_program and does not allow us to control
the order of preference between directories and names.  Create our own
cmFindProgramHelper much like cmFindLibraryHelper but without all the
find_library-specific parts.

Source/cmFindProgramCommand.h

@@ -54,6 +54,7 @@ public:
 
 private:
   std::string FindProgram();
+  std::string FindNormalProgram();
   std::string FindAppBundle();
   std::string GetBundleExecutable(std::string bundlePath);