Introduces the new PIF builder for SwiftBuild (#8405)

Introduces the new PIF builder for packages in `SwiftBuildSupport`.

### Motivation:

This continues the work started by #8271,
introducing now our new *PIF builder* for packages. As expected, this
will happen in the new `SwiftBuildSupport` target, introduced to support
the just released **Swift Build** project.

### Modifications:

This 1st PR *only* includes the new code in the corresponding
`SwiftBuildSupport/PIFPackageBuilder*.swift` source files. The code
isn't actually being used (or unit tested) just yet.

The code still uses the older `SWBProjectModel.PIF` API to build the PIF
data model. In an upcoming PR, I will be migrating this over to the
newer `SwiftBuild.ProjectModel` API, and then finally replacing the
original code to actually use the PIF builder to send package build
information to Swift Build.

PS. I did run `SwiftFormat` in all new code, as indicated by the
contributors guide.

### Result:

This marks the start of our *package PIF builder* unification (i.e.,
between Xcode and SwiftPM).

Tracked by rdar://147767772.

Author: pmattos

Sources/PackageGraph/Resolution/ResolvedModule.swift

@@ -151,7 +151,7 @@ public struct ResolvedModule {
         self.underlying.sources
     }
 
-    let packageIdentity: PackageIdentity
+    package let packageIdentity: PackageIdentity
 
     /// The underlying module represented in this resolved module.
     public let underlying: Module

Sources/SwiftBuildSupport/BuildSystem.swift

@@ -14,7 +14,7 @@ extension BuildSubset {
     var pifTargetName: String {
         switch self {
         case .product(let name, _):
-            PackagePIFProjectBuilder.targetName(for: name)
+            _PackagePIFProjectBuilder.targetName(for: name)
         case .target(let name, _):
             name
         case .allExcludingTests:

Sources/SwiftBuildSupport/CMakeLists.txt

@@ -7,9 +7,15 @@
 # See http://swift.org/CONTRIBUTORS.txt for Swift project authors
 
 add_library(SwiftBuildSupport STATIC
+  BuildSystem.swift
+  PackagePIFBuilder.swift
+  PackagePIFBuilder+Helpers.swift
+  PackagePIFBuilder+Plugins.swift
+  PackagePIFProjectBuilder.swift
+  PackagePIFProjectBuilder+Modules.swift
+  PackagePIFProjectBuilder+Products.swift
   PIF.swift
   PIFBuilder.swift
-  BuildSystem.swift
   SwiftBuildSystem.swift)
 target_link_libraries(SwiftBuildSupport PUBLIC
   Build

Sources/SwiftBuildSupport/PIFBuilder.swift

@@ -120,7 +120,7 @@ public final class PIFBuilder {
             let sortedPackages = self.graph.packages
                 .sorted { $0.manifest.displayName < $1.manifest.displayName } // TODO: use identity instead?
             var projects: [PIFProjectBuilder] = try sortedPackages.map { package in
-                try PackagePIFProjectBuilder(
+                try _PackagePIFProjectBuilder(
                     package: package,
                     parameters: self.parameters,
                     fileSystem: self.fileSystem,
@@ -247,7 +247,7 @@ class PIFProjectBuilder {
     }
 }
 
-final class PackagePIFProjectBuilder: PIFProjectBuilder {
+final class _PackagePIFProjectBuilder: PIFProjectBuilder {
     private let package: ResolvedPackage
     private let parameters: PIFBuilderParameters
     private let fileSystem: FileSystem
@@ -1074,7 +1074,7 @@ final class AggregatePIFProjectBuilder: PIFProjectBuilder {
         allIncludingTestsTarget.addBuildConfiguration(name: "Debug")
         allIncludingTestsTarget.addBuildConfiguration(name: "Release")
 
-        for case let project as PackagePIFProjectBuilder in projects where project.isRootPackage {
+        for case let project as _PackagePIFProjectBuilder in projects where project.isRootPackage {
             for case let target as PIFTargetBuilder in project.targets {
                 if target.productType != .unitTest {
                     allExcludingTestsTarget.addDependency(
@@ -1555,7 +1555,7 @@ extension ResolvedProduct {
     var pifTargetGUID: PIF.GUID { "PACKAGE-PRODUCT:\(name)" }
 
     var mainTarget: ResolvedModule {
-        modules.first { $0.type == underlying.type.targetType }!
+        modules.first { $0.type == underlying.type._targetType }!
     }
 
     /// Returns the recursive dependencies, limited to the target's package, which satisfy the input build environment,
@@ -1612,7 +1612,7 @@ extension Module {
 }
 
 extension ProductType {
-    var targetType: Module.Kind {
+    var _targetType: Module.Kind {
         switch self {
         case .executable:
             .executable