Fix the suffix of dependency urls in Package.swift

[Kyome22]

Bug/issue #, if applicable:

Summary

I unified the URLs of external packages defined in Package.swift's dependencies to have suffix .git.
In the Swift Package Manager ecosystem, it is often better to have .git at the end of the repository URL.

Dependencies

n/a

Testing

n/a

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• [n/a] Added tests
• Ran the ./bin/test script and it succeeded
• [n/a] Updated documentation if necessary

[omochi]

The reasons why it’s better to include .git are explained in detail here.
apple/swift-algorithms#194

[d-ronnqvist]

These two reasons from the linked PR make this worthwhile in my opinion, even if it sounds like SwiftPM successfully resolves the dependencies regardless.

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'.

[d-ronnqvist]

@swift-ci please test