Rename CMake/pkg-config target Catch2Main to Catch2WithMain

This describes the reality better, as it also links in the rest
of Catch2.

The on-disk name of the static library remains just `Catch2Main`,
as that is what it is -- single main function -- and on-disk artifacts
cannot describe link dependencies.

examples/CMakeLists.txt

@@ -52,7 +52,7 @@ set(ALL_EXAMPLE_TARGETS
 )
 
 foreach( name ${ALL_EXAMPLE_TARGETS} )
-    target_link_libraries( ${name} Catch2 Catch2Main )
+    target_link_libraries( ${name} Catch2 Catch2WithMain )
     set_property(TARGET ${name} PROPERTY CXX_STANDARD 14)
     set_property(TARGET ${name} PROPERTY CXX_EXTENSIONS OFF)
 endforeach()