Support overriding generated Crate library targets

[adincebic]

Adding override_target_lib annotation option to allow for replacement of generated library target. For example:

crate.annotation(
    crate = "itoa",
    override_target_lib = "//:my_itoa",
)

NOTE: This is supported by vanilla rules_rust though they allow for overriding other stuff besides library targets.

[adincebic]

@cerisier @dzbarsky