build: use inheritance properly for is_internal
build: use inheritance properly for is_internal

Calling `isinstance(self, X)` is an anti-pattern, we should just be using
inheritance for this, letting the `StaticLibrary` override the method, and
having the base class always return `False`.

mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson