[repro] module maps created for or on objc_library do not propagate to transitive objc_library

this is a repro of an issue i've identified while trying to move to `mixed_language_library` from our custom macro. Regardless of how `module_map` is hinted in `swift_interop_info`, generated, or otherwise attached to an `objc_library`, out-of-the-box these module maps are not propagated to downstream `objc_library` in a `objc -> swift -> objc` arrangement. this is especially problematic in scenarios where PrintAsClang, going by its own uncontrollable heuristics, inserts an `@import` line for the upstream objc library in a generated header.

this is something that needs to change in either `swift_clang_module_aspect` or `objc_library` to discover or otherwise propagate the upstream module maps, as this simple example does not work without customization of the rules or introducing manual dependencies on modulemaps (which isn't possible if relying on the generated modulemaps from the aspect).

Author: aaronsky

examples/apple/objc_interop_modulemap/OIPrintStream.h

@@ -14,11 +14,16 @@
 
 #import <Foundation/Foundation.h>
 
+@protocol Stupid
+@end
+
 /** A very contrived interface for writing strings to a file handle. */
-@interface OIPrintStream : NSObject
+@interface OIPrintStream<PrintType> : NSObject
 
 - (nonnull instancetype)initWithFileHandle:(nonnull NSFileHandle *)fileHandle;
 
+- (void)print:(nonnull PrintType)message;
+
 - (void)printString:(nonnull NSString *)message;
 
 @end

examples/apple/objc_interop_modulemap/OIPrintStream.m

@@ -25,6 +25,9 @@ - (instancetype)initWithFileHandle:(nonnull NSFileHandle *)fileHandle {
   return self;
 }
 
+- (void)print:(nonnull id)message {
+}
+
 - (void)printString:(nonnull NSString *)message {
   NSData *data = [message dataUsingEncoding:NSUTF8StringEncoding];
   [_fileHandle writeData:data];

examples/apple/objc_interop_modulemap/Printer.swift

@@ -15,17 +15,24 @@
 import Foundation
 import examples_apple_objc_interop_modulemap_PrintStream
 
+@objc public protocol MyStupid {
+}
+
 @objc(OIPrinter)
 public class Printer: NSObject {
 
-  private let stream: OIPrintStream
+  private let stream: OIPrintStream<MyStupid>
   private let prefix: String
 
   @objc public init(prefix: NSString) {
     self.stream = OIPrintStream(fileHandle: .standardOutput)
     self.prefix = prefix as String
   }
 
+  @objc public func stream(_ thing: MyStupid) -> OIPrintStream<MyStupid> {
+    return stream
+  }
+
   @objc public func print(_ message: NSString) {
     stream.print("\(prefix)\(message)")
   }