module Pod

REASON:

When a pod only have vendored library, and the 'use_modular_header' is on, cocoapods won't generate a modulemap (so module is not working), beacause it consider it as “should_build? == false”.

It's a bug of cocoapods.

We fix it by changing the `install!` method:

when a pod is (target.should_build? == true && target.uses_modular_headers?),
we treat it differently: like nomarl pods,
   - create_umbrella_header
   - create_module_map