Add a check for an empty package interface path

Update diags

Author: elsh

include/swift/AST/DiagnosticsSema.def

@@ -1110,7 +1110,7 @@ ERROR(in_package_module_not_compiled_from_source_or_package_interface,none,
 
 WARNING(in_package_module_not_compiled_locally,none,
         "module %0 is in package %1 but was loaded from SDK; "
-        "modules of the same package should be built locally: %2",
+        "modules of the same package should be built locally from source only: %2",
         (Identifier, Identifier, StringRef))
 
 ERROR(import_restriction_conflict,none,

lib/Serialization/ScanningLoaders.cpp

@@ -70,7 +70,7 @@ std::error_code SwiftModuleScanner::findModuleFilesInDirectory(
   // Use package.swiftinterface if it exists and its package-name applies to
   // the importer module.
   auto PkgInPath = BaseName.getPackageInterfacePathIfInSamePackage(fs, Ctx).value_or("");
-  if (fs.exists(PkgInPath)) {
+  if (!PkgInPath.empty() && fs.exists(PkgInPath)) {
     InPath = PkgInPath;
   } else {
     // If not in package, use the private interface file if exits.

lib/Serialization/SerializedModuleLoader.cpp

@@ -641,7 +641,7 @@ SerializedModuleBaseName::findInterfacePath(llvm::vfs::FileSystem &fs, ASTContex
   // Check if a package interface exists and if the package name applies to
   // the importer module.
   auto pkgPath = getPackageInterfacePathIfInSamePackage(fs, ctx).value_or("");
-  if (fs.exists(pkgPath))
+  if (!pkgPath.empty() && fs.exists(pkgPath))
     return pkgPath;
 
   // If above fails, use the existing logic as fallback.

test/Sema/import_package_module_from_sdk.swift

@@ -29,7 +29,7 @@
 package func log(level: Int) {}
 
 //--- Client1.swift
-import LibInSDK // expected-warning {{module 'LibInSDK' is in package 'libPkg' but was loaded from SDK; modules of the same package should be built locally}}
+import LibInSDK // expected-warning {{module 'LibInSDK' is in package 'libPkg' but was loaded from SDK; modules of the same package should be built locally from source only}}
 
 func someFunc() {
   log(level: 1)