[PubgrubTests] Replace OrderedDictionary with KeyValuePairs in DependencyGraphBuilder.serve

[One of the callers](https://github.com/apple/swift-package-manager/blob/20eba126ffa32088abb46d642bd0481dbae212ef/Tests/PackageGraphTests/PubgrubTests.swift#L472-L475) of the function passes in a dictionary literal with duplicate keys. `TSCBasic.OrderedDictionary` preserves [the final](https://github.com/apple/swift-tools-support-core/blob/21a79185b2ea8f89b9253ed8cdf2338bf564c499/Sources/TSCBasic/OrderedDictionary.swift#L97-L99) of all duplicate keys' values in a dictionary literal. The logic in this function suggests that duplicate keys can be allowed, and `KeyValuePairs` does exactly that.

Author: WowbaggersLiquidLunch

Tests/PackageGraphTests/PubgrubTests.swift

@@ -2166,7 +2166,7 @@ class DependencyGraphBuilder {
         _ package: String,
         at version: Version,
         toolsVersion: ToolsVersion? = nil,
-        with dependencies: OrderedDictionary<String, OrderedDictionary<String, (PackageRequirement, ProductFilter)>> = [:]
+        with dependencies: KeyValuePairs<String, OrderedDictionary<String, (PackageRequirement, ProductFilter)>> = [:]
     ) {
         serve(package, at: .version(version), toolsVersion: toolsVersion, with: dependencies)
     }
@@ -2175,7 +2175,7 @@ class DependencyGraphBuilder {
         _ package: String,
         at version: BoundVersion,
         toolsVersion: ToolsVersion? = nil,
-        with dependencies: OrderedDictionary<String, OrderedDictionary<String, (PackageRequirement, ProductFilter)>> = [:]
+        with dependencies: KeyValuePairs<String, OrderedDictionary<String, (PackageRequirement, ProductFilter)>> = [:]
     ) {
         let packageReference = reference(for: package)
         let container = self.containers[package] ?? MockContainer(package: packageReference)