meson: rename libsystemd_internal to libsystem_static

meson: rename libsystemd_internal to libsystem_static

We already use the "_static" suffix for libshared_static ("shared" is the name
of the library, "static" is the format) and other libs, so let's rename for
consistency.

Also change libsystemd_static_sources to libsystemd_sources, since the same
list is used for both and shorter is better.