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.

bootstrap

@@ -468,6 +468,7 @@ CMAKE_CXX_SOURCES="\
   cmGccDepfileLexerHelper \
   cmGccDepfileReader \
   cmReturnCommand \
+  cmPackageInfoReader \
   cmPlaceholderExpander \
   cmPlistParser \
   cmRulePlaceholderExpander \