[Dependency Scanner] Do not re-use prior source dependencies in successive scans

If a scanner instance is used for multiple scans, the `SwiftDependencyScanningService` will have collected prior scans' main source modules' dependency info.
Suppose the scanner first scans a source module `A` and records its dependencies. If a successive scan of some source module `B` finds that it depends on `A`, it would get a cache hit on the source module from a prior scan and re-use dependency information. This change no longer allows such re-use of the prior source module scan dependencies. Said prior scan may have had an entirely different context and therefore different set of dependencies on `A` than may be visible otherwise. Instead, all scanning actions must rely on such modules being re-discovered on the filesystem.

lib/AST/ModuleDependencies.cpp

@@ -422,7 +422,19 @@ ModuleDependenciesCache::ModuleDependenciesCache(
 Optional<const ModuleDependencyInfo*>
 ModuleDependenciesCache::findDependency(
     StringRef moduleName, Optional<ModuleDependencyKind> kind) const {
-  return globalScanningService.findDependency(moduleName, kind);
+  auto optionalDep = globalScanningService.findDependency(moduleName, kind);
+  // During a scan, only produce the cached source module info for the current module
+  // under scan.
+  if (optionalDep.hasValue()) {
+    auto dep = optionalDep.getValue();
+    if (dep->getAsSwiftSourceModule() &&
+        moduleName != mainScanModuleName &&
+        moduleName != "DummyMainModuleForResolvingCrossImportOverlays") {
+      return None;
+    }
+  }
+
+  return optionalDep;
 }
 
 bool ModuleDependenciesCache::hasDependency(