Remove build warnings

[bdewey]

We had hundreds of warnings from:

1. Including header files from openssl / libssh in the built xcframework that weren't covered by the libgit2 umbrella header
2. Not setting the right target SDK when building

Resolving #1 meant changing the build strategy. The dependencies are built into separate install directories, and then their resulting *.a files are all combined with libgit2.a. None of the dependent headers are now included in the framework.