Use SPM API diff checker (#351)
Use SPM API diff checker (#351)

### Motivation:

SPM has built in functionality to check the API of modules against a target git treeish. We can use this to simplify our `check_no_api_breakages.sh` script. Closes https://github.com/apple/swift-nio/issues/1239

### Modifications:

This PR, exchanges the direct calls to Swift's API checker with the new SPM `diagnose-api-breaking-changes` tool. This allows us to get rid of the manual module parsing, build invocations and result comparisons.

### Result:

We are now using SPMs `diagnose-api-breaking-changes` to check for breaking changes.
apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl,apple/swift-nio-ssl