Order of module imports affect availability of Units

[domdom]

Describe the bug
Importing the 'flecs::stats' module before the 'flecs::units' module results in an error message when trying to register a component member as having a given Unit type.

To Reproduce
Consider the following code:

#include <flecs.h>

int main() {

    flecs::world world;
    world.import<flecs::stats>();
    world.import<flecs::units>();

    struct Foo {
        float x;
    };

    world.component<Foo>().member<flecs::units::length::Meters>("x", &Foo::x);

    return 0;
}

Running that code emits the following:

error: meta.c: 392: entity 'Meters' for member 'x' is not a unit

Swapping the order of those two imports fixes the issue.

NB: This is running on v4.0.0 of flecs.