find_package: Actually find .cps files

Add a helper class to read CPS files. Use this to teach find_package how
to consider and accept .cps files in its search. (Note that no version
testing is performed at this time.) Add a simple test that we can find a
package from a .cps file and correctly extract the version information.

Note that this doesn't actually import anything from CPS yet.

Source/cmFindPackageCommand.h

@@ -7,6 +7,7 @@
 #include <cstddef>
 #include <functional>
 #include <map>
+#include <memory>
 #include <set>
 #include <string>
 #include <utility>
@@ -17,6 +18,7 @@
 #include <cm3p/kwiml/int.h>
 
 #include "cmFindCommon.h"
+#include "cmPackageInfoReader.h"
 #include "cmPolicies.h"
 
 // IWYU insists we should forward-declare instead of including <functional>,
@@ -283,6 +285,8 @@ private:
   };
   std::vector<ConfigFileInfo> ConsideredConfigs;
 
+  std::unique_ptr<cmPackageInfoReader> CpsReader;
+
   friend struct std::hash<ConfigFileInfo>;
 };