Add find_package overrides that also work in CONFIG mode (#498)

[patstew]

CPM currently adds a FindFoo.cmake to the module path to override subsequent find_package calls. This makes sure that works when the find_package call is in CONFIG mode as well. Some of my dependencies use this in their CMakeLists.txt. This is equivalent to the OVERRIDE_FIND_PACKAGE argument to FetchContent, we could add that option to CPMAddPackage, but it seems like this is the intended default behaviour of CPM?

The include(....-extra.cmake) bits are something FetchContent does that's also useful here. They should maybe be added to the Find*.cmake version too?

Fixes #498