git2 framework+module

[phatblat]

This is an experiment to see if creating a true module resolves the following issues:

• 🚫 Include of non-modular header inside framework module 'ObjectiveGit'
• ⚠️ Umbrella header for module 'ObjectiveGit' does not include header '/git2/cred_helpers.h'

While it is possible to turn the libgit2 static library into a module, the result is an extra file that must be placed correctly alongside the header files. Wrapping all these up in a framework simplifies things for client projects.

[phatblat]

Projects trying to use ObjectiveGit without enabling CLANG_ALLOW_NON_MODULAR_INCLUDES_IN_FRAMEWORK_MODULES see errors like this:

What's odd about this setting is that once you have a clean build after it is enabled, these errors will not come back until you:

1. Clean the project
2. Delete DerivedData/ModuleCache
3. Restart Xcode

[phatblat]

The umbrella header warnings look like this:

These are mostly just annoying, but could cause problems if the referenced headers are needed by a client. It's basically saying "this header is part of the module, but not included in the umbrella header".

[phatblat]

I suspect the "non-modular header errors" were only showing up in Swift apps because Swift must use modules to import symbols, whereas Objective-C #import uses the old mechanism (see libgit2#436). I never tested it, but I suspect Objective-C apps using @import ObjectiveGit; would have these module errors as well,

[phatblat]

This work has been moved to libgit2#570