Only pass --enable-parseable-module-interfaces when building PackageDescription and PackagePlugin.

This avoids a bunch of probably-harmless-but-still-worrisome warnings that happen when trying to build all of the rest of SwiftPM this way.

Author: abertelrud

Utilities/bootstrap

@@ -629,8 +629,6 @@ def build_swiftpm_with_swiftpm(args, integrated_swift_driver):
     if integrated_swift_driver:
         swiftpm_args.append("--use-integrated-swift-driver")
 
-    swiftpm_args.append("--enable-parseable-module-interfaces")
-    
     # Build SwiftPM, including libSwiftPM, all the command line tools, and the current variant of PackageDescription.
     call_swiftpm(args, swiftpm_args)
 
@@ -648,6 +646,11 @@ def build_swiftpm_with_swiftpm(args, integrated_swift_driver):
     mkdir_p(lib_dir)
 
     symlink_force(os.path.join(args.bootstrap_dir, "pm"), os.path.join(lib_dir, "pm"))
+    
+    # Build PackageDescription and PackagePlugin with parseable module interfaces enabled.
+    call_swiftpm(args, swiftpm_args + ["--product", "PackageDescription", "--enable-parseable-module-interfaces"])
+    call_swiftpm(args, swiftpm_args + ["--product", "PackagePlugin", "--enable-parseable-module-interfaces"])
+    
 
 def call_swiftpm(args, cmd, cwd=None):
     """Calls a SwiftPM binary with the necessary environment variables and flags."""