Ability to write out package manifests as source code, providing more flexibility for things like mechanical package manifest generation.

Note that this is _not_ suitable for editing the package manifest, since that would need to preserve whitespace and comments.  Rather, this is for things like automatically generating packages from test sets, or for generating the initial configuration of packages (e.g. a more flexible `package init`).

rdar://70271810

Author: abertelrud

Sources/PackageModel/CMakeLists.txt

@@ -19,6 +19,7 @@ add_library(PackageModel
   Manifest/SystemPackageProviderDescription.swift
   Manifest/TargetBuildSettingDescription.swift
   Manifest/TargetDescription.swift
+  ManifestSourceGeneration.swift
   ModuleMapType.swift
   Package.swift
   PackageReference.swift