[Serialization] Attempt to load transitive impl-only dependencies on testable imports

Implementation-only dependencies may be referenced from internal decls.
When that module is imported as @testable, clients see the internal
decls and may fail accessing them if the transitive implementation-only
dependencies are not loaded.

Let's consider such transtive implementation-only dependencies as
optional for @testable imports. As such, the compiler will attempt to
load them for test targets, and won't fail if the dependency is missing.

We can make these dependencies required for non-public imports, but it
could be project breaking to do so for implementation-only dependencies.
Considering them as optional is a decent compromise.

rdar://79459263

lib/Serialization/ModuleFileSharedCore.cpp

@@ -1702,7 +1702,7 @@ ModuleFileSharedCore::getTransitiveLoadingBehavior(
   if (dependency.isImplementationOnly()) {
     // Implementation-only dependencies are not usually loaded from
     // transitive imports.
-    if (debuggerMode) {
+    if (debuggerMode || forTestable) {
       // In the debugger, try to load the module if possible.
       // Same in the case of a testable import, try to load the dependency
       // but don't fail if it's missing as this could be source breaking.