[ios][prebuild] refactor header/umbrella/modulemap

To make sure we are exposing the same public API to swift as without prebuilt, this commit changes the follow:

- ModuleMap / Umbrella file is generated from template, not dynamically to align with non-prebuilt (cocoapods)
- Headers are extracted by reading from podspec files and expanding their globs. This is now easy since we can use the podspec_sources function to look for file globs.