That depends; in the case of mathematics libraries they have to be bundled together because of functional dependencies. It's either that or ignoring DRY principles -- which if you do that, you're ignoring the entire point of what breaking things into submodules is intended to do.
Separate interface and implementation. cos and sin might well use common underlying code (by depending on the same module), but they should expose orthogonal interfaces.
That depends; in the case of mathematics libraries they have to be bundled together because of functional dependencies. It's either that or ignoring DRY principles -- which if you do that, you're ignoring the entire point of what breaking things into submodules is intended to do.