Use non-deprecated SPM methods in Package.swift file

[dfed]

• I have updated Purchases SDK to the latest version
• I have read the Contribution Guidelines
• I have searched the Community
• I have read docs.revenuecat.com
• I have searched for existing Github issues

Describe the bug
This library's Package.swift file is currently using the .exact(…) enum rather than the exact: parameter. The former is deprecated, and this causes warning output in adopting libraries.

1. Environment
   1. Platform:
   2. SDK version:
   3. StoreKit version:
      • StoreKit 1 (default on versions <5.0.0. Can be enabled in versions >=5.0.0 with .with(storeKitVersion: .storeKit1))
      • StoreKit 2 (default on versions >=5.0.0)
   4. OS version: any
   5. Xcode version: Xcode 14 and later (could be 13, not sure)
   6. Device and/or simulator:
      • Device
      • Simulator
   7. Environment:
      • Sandbox
      • TestFlight
      • Production
   8. How widespread is the issue. Percentage of devices affected.
2. Debug logs that reproduce the issue. Complete logs with Purchases.logLevel = .verbose will help us debug this issue.

warning: 'purchases-ios-spm': /Users/dfed/source/apple/Apps/.build/checkouts/purchases-ios-spm/Package.swift:20:26: warning: 'package(url:_:)' is deprecated: use specific requirement APIs instead (e.g. use 'branch:' instead of '.branch')
18 |     // Versions 1.4.0 and 1.4.1 are failing to compile, so we are pinning it to 1.3.0 for now
19 |     // https://github.com/RevenueCat/purchases-ios/pull/4216
20 |     dependencies.append(.package(url: "https://github.com/apple/swift-docc-plugin", .exact("1.3.0")))
   |                          `- warning: 'package(url:_:)' is deprecated: use specific requirement APIs instead (e.g. use 'branch:' instead of '.branch')
21 | }
22 | 

3. Steps to reproduce, with a description of expected vs. actual behavior

Add Purchases-iOS (or Purchases-iOS-SPM) dependency. Resolve packages.

4. Other information (e.g. stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, etc.)
   deprecate name attribute on package dependencies swiftlang/swift-package-manager#3641

5. Additional context
   PRs are out to solve this issue:
   Use exact: rather than .exact(…) #4276
   Use exact: rather than .exact(…) purchases-ios-spm#8

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