Switch to ProjectModel API in the new SwiftBuildSupport PIF builder (#8441)

### Motivation:

The goal is to adopt the new `SwiftBuild.ProjectModel` API. This new API
provides a *typesafer* and *modern* way of building PIFs
programmatically.

This continues the work I started in PR #8405, introducing now our new
PIF builder for packages.

### Modifications:

Replaces all `SwiftBuild.PIF` (aka, `SWBProjectModel.PIF`) API usage, in
`PackagePIFBuilder`, with the new `SwiftBuild.ProjectModel` API instead.

### Result:

`PackagePIFBuilder` is now modernized... but still not actually used.
This will come in my next pull request.

Tracked by rdar://147526957.

Author: pmattos

Sources/SwiftBuildSupport/PIFBuilder.swift

@@ -1641,7 +1641,7 @@ private struct PIFBuildSettingAssignment {
     let platforms: [PIF.BuildSettings.Platform]?
 }
 
-extension BuildSettings.AssignmentTable {
+extension PackageModel.BuildSettings.AssignmentTable {
     fileprivate var pifAssignments: [PIF.BuildSettings.MultipleValueSetting: [PIFBuildSettingAssignment]] {
         var pifAssignments: [PIF.BuildSettings.MultipleValueSetting: [PIFBuildSettingAssignment]] = [:]
 
@@ -1679,7 +1679,7 @@ extension BuildSettings.AssignmentTable {
     }
 }
 
-extension BuildSettings.Assignment {
+extension PackageModel.BuildSettings.Assignment {
     fileprivate var configurations: [BuildConfiguration] {
         if let configurationCondition = conditions.lazy.compactMap(\.configurationCondition).first {
             [configurationCondition.configuration]
@@ -1866,7 +1866,7 @@ extension PIF.BuildSettings {
                 .filter { isSupportedVersion($0) }.map(\.description)
         }
 
-        func computeEffectiveTargetVersion(for assignment: BuildSettings.Assignment) throws -> String {
+        func computeEffectiveTargetVersion(for assignment: PackageModel.BuildSettings.Assignment) throws -> String {
             let versions = assignment.values.compactMap { SwiftLanguageVersion(string: $0) }
             if let effectiveVersion = computeEffectiveSwiftVersions(for: versions).last {
                 return effectiveVersion

Sources/SwiftBuildSupport/PackagePIFBuilder+Helpers.swift

@@ -18,9 +18,10 @@ import enum Basics.Sandbox
 import struct Basics.SourceControlURL
 
 #if canImport(SwiftBuild)
-import enum SwiftBuild.PIF
 
-extension PIFPackageBuilder {
+import enum SwiftBuild.ProjectModel
+
+extension PackagePIFBuilder {
     /// Contains all of the information resulting from applying a build tool plugin to a package target thats affect how
     /// a target is built.
     ///