Consider allowing $overrides service autoload

[nfroidure]

Currently, the service is a constant (and is forced to be a constant) https://github.com/nfroidure/knifecycle/blob/main/src/index.ts#L283

The reason for it is that loading a service could lead to different results overtime which is not a good thing.

That said, we could allow to change the $overrides mapping (and autoload it) by checking that the mapping were not used yet and refuse to change it by another mapping that don't fit the previously mapped service names.