build: Add some type annotations to BuildTarget
build: Add some type annotations to BuildTarget

This is not complete, it's just enough for backend/backend.py. A more
completely typing would be more difficult, especially whithout
untangling the layering violation between the build targets and the
interpreter.

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