Move Eigen.hpp out of prim

[SteveBronder]

Description

Right now most files include stan/math/prim/fun/Eigen.hpp before everything else.

#include <stan/math/prim/fun/Eigen.hpp>
#include <stan/math/rev/meta.hpp>
#include <stan/math/rev/core.hpp>
#include <stan/math/rev/fun/value_of.hpp>

But our rules for fwd and rev are the you should always include fwd/rev files before including anything from prim. So we should move Eigen.hpp up to the math folder so it is out of prim.

Current Version:

v4.9.0

[WardBrian]

I think this is a good idea, but we should probably leave Eigen.hpp in prim/fun, that includes whatever the new location is, because it is used outside of just this repo:

https://github.com/search?q=org%3Astan-dev+stan%2Fmath%2Fprim%2Ffun%2FEigen.hpp+NOT+repo%3Astan-dev%2Fmath&type=code

[SteveBronder]

Yes that's reasonable. We can have the prim/fun file just include the other file