[Traits] Package trait description and serialization (#7669)

### Motivation:

This PR is the first one to land the package traits feature in SwiftPM. It focuses on the new public APIs and serialization of them.

### Modifications:

This PR contains:
- The `PackageDescription` APIs used to define traits, optional dependencies based on traits and build settings based on traits;
- The serialization logic to encode the trait specific changes in the package description;
- The deserialization logic to decode the trait specific changes into the `PackageModel`;
- Validation logic for traits;
- Tests for all of the above.

I gated the new APIs behind both the `999` availability and an `@_spi(ExperimentalTraits)` to allow us to land this on `main` before the proposal is accepted.

### Result:

We can now define a package with traits 🚀

Author: FranzBusch

Sources/Commands/PackageCommands/AddDependency.swift

@@ -136,7 +136,8 @@ extension SwiftPackageCommand {
                 nameForTargetDependencyResolutionOnly: nil,
                 location: location,
                 requirement: requirement,
-                productFilter: .everything
+                productFilter: .everything,
+                traits: []
             )
 
             let editResult = try AddPackageDependency.addPackageDependency(

Sources/PackageDescription/BuildSettings.swift

@@ -59,10 +59,13 @@ public struct BuildSettingCondition: Sendable {
     let platforms: [Platform]?
     /// The applicable build configuration for this build setting condition.
     let config: BuildConfiguration?
+    /// The applicable traits for this build setting condition.
+    let traits: Set<String>?
 
-    private init(platforms: [Platform]?, config: BuildConfiguration?) {
+    private init(platforms: [Platform]?, config: BuildConfiguration?, traits: Set<String>?) {
         self.platforms = platforms
         self.config = config
+        self.traits = traits
     }
 
     @available(_PackageDescription, deprecated: 5.7)
@@ -71,7 +74,24 @@ public struct BuildSettingCondition: Sendable {
         configuration: BuildConfiguration? = nil
     ) -> BuildSettingCondition {
         precondition(!(platforms == nil && configuration == nil))
-        return BuildSettingCondition(platforms: platforms, config: configuration)
+        return BuildSettingCondition(platforms: platforms, config: configuration, traits: nil)
+    }
+
+    /// Creates a build setting condition.
+    ///
+    /// - Parameters:
+    ///   - platforms: The applicable platforms for this build setting condition.
+    ///   - configuration: The applicable build configuration for this build setting condition.
+    ///   - traits: The applicable traits for this build setting condition.
+    @_spi(ExperimentalTraits)
+    @available(_PackageDescription, introduced: 999.0)
+    public static func when(
+        platforms: [Platform]? = nil,
+        configuration: BuildConfiguration? = nil,
+        traits: Set<String>? = nil
+    ) -> BuildSettingCondition {
+        precondition(!(platforms == nil && configuration == nil && traits == nil))
+        return BuildSettingCondition(platforms: platforms, config: configuration, traits: traits)
     }
 
     /// Creates a build setting condition.
@@ -81,23 +101,23 @@ public struct BuildSettingCondition: Sendable {
     ///   - configuration: The applicable build configuration for this build setting condition.
     @available(_PackageDescription, introduced: 5.7)
     public static func when(platforms: [Platform], configuration: BuildConfiguration) -> BuildSettingCondition {
-        BuildSettingCondition(platforms: platforms, config: configuration)
+        BuildSettingCondition(platforms: platforms, config: configuration, traits: nil)
     }
 
     /// Creates a build setting condition.
     ///
     /// - Parameter platforms: The applicable platforms for this build setting condition.
     @available(_PackageDescription, introduced: 5.7)
     public static func when(platforms: [Platform]) -> BuildSettingCondition {
-        BuildSettingCondition(platforms: platforms, config: .none)
+        BuildSettingCondition(platforms: platforms, config: .none, traits: nil)
     }
 
     /// Creates a build setting condition.
     ///
     /// - Parameter configuration: The applicable build configuration for this build setting condition.
     @available(_PackageDescription, introduced: 5.7)
     public static func when(configuration: BuildConfiguration) -> BuildSettingCondition {
-        BuildSettingCondition(platforms: .none, config: configuration)
+        BuildSettingCondition(platforms: .none, config: configuration, traits: nil)
     }
 }
 

Sources/PackageDescription/CMakeLists.txt

@@ -15,11 +15,13 @@ add_library(PackageDescription
   PackageDescriptionSerialization.swift
   PackageDescriptionSerializationConversion.swift
   PackageDependency.swift
+  PackageDependencyTrait.swift
   PackageRequirement.swift
   Product.swift
   Resource.swift
   SupportedPlatforms.swift
   Target.swift
+  Trait.swift
   Version.swift
   Version+StringLiteralConvertible.swift)