Pass REQUIRED to find_package(Grantlee).

I wasn't doing it before so that the user would get a complete list of
missing packages from feature_summary, instead of a failure halfway
through. But templates/CMakeLists.txt calls a macro in grantlee so it fails
halfway through anyway.
Remove duplicated call to feature_summary.

It's already being called in the root CMakeLists.txt, calling it again
in plugins/ causes double output.
Move __pycache__ to excluded folders, it's not a file