Rename MlirExecutionEngine lookup to lookupPacked

The purpose of the change is to make clear whether the user is
retrieving the original function or the wrapper function, in line with
the invoke commands. This new functionality is useful for users that
already have defined their own packed interface, so they do not want the
extra layer of indirection, or for users wanting to the look at the
resulting primary function rather than the wrapper function.

All locations, except the python bindings now have a `lookupPacked`
method that matches the original `lookup` functionality. `lookup`
still exists, but with new semantics.

- `lookup` returns the function with a given name. If `bool f(int,int)`
is compiled, `lookup` will return a reference to `bool(*f)(int,int)`.
- `lookupPacked` returns the packed wrapper of the function with the
given name. If `bool f(int,int)` is compiled, `lookupPacked` will return
`void(*mlir_f)(void**)`.

Differential Revision: https://reviews.llvm.org/D114352