Specify dependency URL in consistent fashion

[gwynne]

All this does is add the .git suffix to the package URL for swift-numerics. Why do this?

• Consistency. A majority of the other Apple-published Swift ecosystem packages (swift-logging, swift-crypto, swift-certificates, swift-asn1, swift-collections, swift-docc, swift-nio, swift-argument-parser, swift-atomics, and so on) use the .git-suffixed form, if not exclusively, then more often than not.
• It makes a difference. If, for example, the same dependency is specified more than once in a given dependency graph and the use of the suffix is inconsistent, SwiftPM does take notice (although the message is only shown when -v is in use):

  info: 'swift-algorithms': dependency on 'swift-numerics' is represented by similar locations
  ('https://github.com/apple/swift-numerics.git' and 'https://github.com/apple/swift-numerics')
  which are treated as the same canonical location 'github.com/apple/swift-numerics'.
  

• It makes a noticeable difference; which form ends up in Package.resolved tends to change between SwiftPM versions, as well as when the file is updated by Xcode versus the commandline tooling. For projects which commit Package.resolved to their Git repos, this results in annoyances like "unclean Git status with no changes made" or "why does Package.resolved change back and forth in every single commit on this branch?"
• Doing this has no meaningful impact in any functional sense; there's no need to tag a release for this change, it'll just get picked up next time something happens that does warrant a release.

[natecook1000]

@swift-ci Please test