Module Maps for Obj-C/C/C++ include too many files

I noticed this with https://github.com/facebook/yoga

```
filegroup(
name = "yoga_hdrs",
srcs = glob(
[
"pod_support/Headers/Public/**/*.h"
],
exclude_directories = 1
),
visibility = [
"//visibility:public"
]
)
gen_module_map(
"yoga",
"yoga_module_map",
"yoga",
[
"yoga_hdrs"
]
)
```

It seems like `pod_support/Headers/Public/` contains too many files (every yoga header, public or private), given that the podspec lists only three public headers: https://github.com/facebook/yoga/blob/master/Yoga.podspec#L28.