update diag message

Author: elsh

include/swift/AST/DiagnosticsSema.def

@@ -1007,7 +1007,7 @@ ERROR(module_not_compiled_for_private_import,none,
 
 ERROR(package_module_not_compiled_from_source,Fatal,
       "module %0 is a package module but was built from interface; "
-      "it must be compiled from source instead",
+      "package modules can only be loaded when built from source",
       (Identifier))
 
 ERROR(import_restriction_conflict,none,

test/Serialization/load_package_module.swift

@@ -9,7 +9,7 @@
 
 // RUN: not %target-swift-frontend -module-name Client1 %t/Client1.swift -emit-module -emit-module-path %t/Client1.swiftmodule -enable-library-evolution -package-name myPkg -I %t 2> %t/result.output
 // RUN: %FileCheck %s -check-prefix CHECK-CLIENT < %t/result.output
-// CHECK-CLIENT: module 'LibFromInterface' is a package module but was built from interface; it must be compiled from source instead
+// CHECK-CLIENT: module 'LibFromInterface' is a package module but was built from interface; package modules can only be loaded when built from source
 
 // RUN: %target-swift-frontend -module-name LibFromSource -emit-module -emit-module-path %t/LibFromSource.swiftmodule -parse-as-library %t/Lib.swift -package-name myPkg
 // RUN: test -f %t/LibFromSource.swiftmodule