Support swift package manager

[imjoehaines]

Goal

This PR adds support for Swift Package Manager

This involved moving the header files to a new include directory to match the expected convention for C projects in SPM. This isn't absolutely necessary, for example we could use symlinks instead, but this felt like the cleanest solution

Resolves #407

Changeset

Most of the changes are moving header files into include/Bugsnag. The podspec and Xcode project file have been updated with these new paths too

I've removed some unnecessary imports in BugsnagConfiguration.m. This was part of an effort to split up the package into sup-dependencies which was ultimately abandoned, but I figured this was a useful cleanup anyway. Similarly, BugsnagPlugin was missing an import which causes issues if it's built before the header it relies on is included, so I added that too

Tests

I've tested this in our Swift example app and new Swift projects

Carthage and Cocoapods are tested on CI and I've manually tested them in the example projects & static build. I've also tested that our manual install instructions still work

[tomlongridge]

The doc target on the Makefile uses the public_header_files entry to generate docs for each entry. I don't think this will now work with the wildcard - can you check and find an alternative? (It's definitely better with the wildcard, so we should fix the step not the podspec file.)

[imjoehaines]

make doc works fine with the wildcard as it uses Dir.glob 🙂 Someone must have seen this coming!

[tomlongridge]

LGTM

[twometresteve]

Can we change this to "Version Up to next major 6.1.0 < 7" or similar before merging - so that it's correct after release.