Add support for Swift Package Manager

[mattt]

Initial work to add Swift support to Dependabot.

This is currently a work-in-progress, but I'd welcome any feedback on what we have so far.

Fix #1245

[apps4everyone]

Hey @mattt thx for creating this draft. Did you got any feedback? Is there anything I could help you with? (testing?) Would really love to have this feature available.

[elu-dawn]

Thanks a lot for this initial work. Is there anything we can do to help get this moving? We'd also love to have SPM support in Dependabot.

[FranciscoPereira]

Many thanks!! I'm also available to help @mattt.

[tomerd]

if you can control the FROM consider using swift.org published docker images instead of installing the toolchain from tarball

[tomerd]

otherwise, swift.org will so publish RPMs and Debs which could make this easier

[lucasilverentand]

Hi, I was just wondering if this feature is still in the works? I think it would make many iOS developers on GitHub very happy. Did I miss any developments on this? Also I would be very happy to help with any development if needed.

[hernangonzalez]

Has this been abandoned @mattt ? Thanks

[jeffwidman]

If / when we do pick this up, there's also prior art over in #5562 that may be useful. That PR also has some NotImplemented bits so neither one is completely ready, but that one does look to have some additional test cases and some features that haven't been completed yet in this PR etc. So worth a skim to see if any commits should be cherry-picked into this.